In most space opera settings, aliens are human-sized. A "giant" alien might be all of 9' tall; small ones tend to bottom out at 3'. In media with budget constraints, this is fine. You need to be able to fit an actor into the rubber suit. But RPGs have no budgets.
For reasons of convenience, humans might associate with similarly-scaled aliens (with similar atmosphere, gravity, pressure, etc. requirements), but there's no reason that has to be the absolute limit. "Bleeds green blood and has two hearts" is barely weird biology.
Note: the obscure string of letters and numbers attached to each species is explained in this post.The inspiration for this post was a comment from Nobodi12.
But this categories face the challenge of " If everyone has to wear miniature spaceships to visit each other, it's not space opera. "And they are right. Anything significantly bigger than a human is a threat, or at the very least unnerving. (Dragons, whales, etc.) Anything significantly smaller is adorable or possibly creepy (spiders, mice, butterflies, etc.). We're very focused on our own scale.
If we can't survive in the same atmosphere the kicks one get out of aliens are much minor. As a player I would have less fun due to limited interaction with aliens. It makes barriers and making interesting NPCs is already hard. Worse if they have to be PCs. Size as well, anything more then 1 order of magnitude different then a human becomes a McGuffin/Boss monster and not something you routinely interact with.
Let's look at a few examples from the 1c (1 cognitive-speed-equivalent to human, i.e. people you can carry on a real-time conversation with) range. How could a GM use them as NPCs? How could they use them as PCs?
The Central VreenG - H2 - -100/200/600oC - 0/0.02/0.3atm - 0/2/5g - N# - 1000m - 1c - IS - BR5
The Central Vreen resemble transparent sea jellies. A typical mid-life roving unit (as opposed to a juvenile, an instar, or a sedentary late-life retirement unit) is around 1km long. They are therefore at the upper end of individual-social life in general. (Colony-units, vacuum-dwelling film-envelopes, and some geomorphs can grow to orders of magnitude larger.)
They are very wealthy for three reasons. First, any Central Vreen ship is, by necessity, enormous. Decommissioned personal shuttles are occasionally used as bulk transports or space stations. Second, they live for a very long time by most galactic standards, and can molt to a previous stage if the situation requires it, giving them effective immortality. They build structures, both political and economic, to last. And third, they have a knack for social adaptability, fitting their requirements to the needs or biases of other species.
(A fourth theory, put forward by a few eccentric xenobiologists, states that the Central Vreen like to acquire and cultivate smaller life forms as a throwback to their original forms as the backbone of living gas giant "reef systems". A healthy "reef" has many "fish".)
Their size and atmospheric requirements make face-to-face meetings with most species impossible. They communicate primarily via radio, with colour-shifting skin patterns for emphasis. The Central Vreen use telepresence screens. Their "faces" are a familiar sight to most diplomats, who occasionally forget that the face on the monitor is the size of the average cargo ship.
But they also use drone-units (known as ROVs or Waldoes). They metallic build to-scale replicas of the species they intend to visit, then pilot them via a distant virtual reality dome. Some use telepresence screens to project the Central Vreen's face, others have featureless masks or simple indicators.
A Central Vreen PCs using a Waldoes is, effectively, driving a robot, usually with 360 degree vision (the Central Vreen have full-body photoreceptors) and a tendency to misjudge distances and respective sizes. There's plenty of room to put a human-scale ship on the outside of a Central Vreen hull without anyone noticing, but a Central Vreen ship is comparatively quite fragile.
The OrloG - O2 - -40/30/90oC - 0.1/1.3/3atm - 0/1/3g - P# - 10m - 1c - IS - B
The Orlo are thousand-limbed pale blue beetles the size of a car. Eight gem-like eyes in two rows of four, truly alarming manipulator mouthparts (hidden by a veil), and decorative horns. They are social among their own species, but only after formal introductions and a strict hierarchy is established. Some xenoanthropolgists classify them as forming "#-sized indivisible units" instead, because crews and diplomatic missions can fail completely if a member is removed or added unexpectedly. Other species, thankfully, do not trigger this response.
An Orlo can fit in most human-sized habitats, but smaller space stations and narrow corridors present a problem. They excrete cat-sized semi-parasitic sensory grubs to investigate difficult spaces, then devour them upon their return. The grubs are slightly intelligent, can speak, and can manipulate objects with their mouthparts.
An Orlo PC is large, armoured (to a certain extent), and used to biting their problems to death. The exact limitations of a sensory grub are up to the GM.
The KursleckG - O2 - -40/10/30oC - 0.2/0.9/1.1atm - 0/1.4/4g - P# - 0.1m - 1c - IS - I
Kursleck are melon-sized spheres of dense black prehensile hair with two extensible sensory stalks. Upon joining galactic society, they were chagrined to discover most of the species sharing compatible environments were considerably larger than them, and often completely immune to the sound-based stealth tactics the nearly blind Kursleck took for granted. Still smarting from a few ill-advised wars, the Kursleck are slowly finding their place in the galaxy.
To avoid being trampled, modern Kursleck are suspended beneath floating grav discs. The sight of a black furball with a halo, hovering at head height, and sniffing the air with two antennae can be alarming, but seeing eye-to-eyestalk has advantages.
A Kursleck PC has massive silent movement bonuses (except against sight) and, if they've got a hover-disc, can fly. Terrible eyesight but excellent hearing, and the ability to hold up to 60 ring-sized objects.
HumansG - O2 - -20/20/40oC - 0.3/1/1.3atm - 0/1/3g - P# - 1m - 1c - IS - B
Humans are slightly smaller than average, prefer slightly more gravity, and slightly lower temperatures than their their assigned atmosphere-temperature-gravity band. They are well within the centre of the bell curve; over a hundred species can directly interact with human structures and vice-versa, with a further three hundred species capable of interaction with limited adaptation (extra insulation, limited exposure, scale issues, etc.).
As relative newcomers with a limited technological base, humans tend to use telepresence for most scale-differential interactions, though they can adapt to Waldo piloting fairly easily. Some of their smaller ships are configured to interact with larger gas-giant species directly. Humans have broadly porous social groups, and assign human-like characteristics to inimate objects and non-sentient creatures. If they find a conceptual handle for a new species, they rapidly work out how to interact with them. This can result in disaster, but on average tends to work out positively.
|1d8||Smaller to Larger Interaction Methods|
|1||Appropriately scaled manipulator suit or mini-craft.|
|2||Suspensor platform or hoverchair.|
|3||Natural flight and quick turns.|
|4||Coordinated assembly-stack. Highest ranked on top.|
|5||Stoic acceptance, vivid communication, and high speed.|
|6||Dedicated porter-species. Possibly a host.|
|7||Hired porters and local guides, possibly unaware of their role.|
|8||Holographic adjustable communication-form.|
|1d8||Larger to Smaller Interaction Methods|
|1||ROVs/Waldoes. Uniform models, custom-built, or off-the-shelf adaptations.|
|2||Telepresence and distant guidance.|
|3||Extremely dangerous temporary dimensional compressor.|
|4||Dedicated interpreter-species. Possibly mind-linked.|
|5||Juvenile forms sent to live among other species, growth halted via medicine.|
|6||Growth into starship- or -transport forms. Live inside one.|
|7||Temporary budding or subunit/organ detachment.|
|8||Extruded sensory goop.|