litphoria

a variable size trait for all segments that have a size value

Body height of variable? Would that not be a shape shifter? Or are we implying something else here?

Shapeshifter works for it if you want to generalize it a lot. But size changing does not allow you to change your shape, if that's really all you do. It would also work for people who aren't too picky about which size their character is in each RP if there's nothing better for that, if worded properly. There's also body growth/shrinking stuff, but this would be more explicitly voluntary ability. Personally I'd honestly just leave size blank for variable size, tho, if it's the adjust to each RP rather than being an ability thing, otherwise it's mostly in realm of shrinking/growing activities which already exist I think.

And yes, other sizes being variable would be nice. Like maybe you're an android with almost literal balloon breasts that can be inflated and deflated at will, or by magic where the size of your breasts changes with your mood (both of which are somewhat silly to me, but I'm sure there's people into that). There isn't activities for growing/shrinking each body aspect people might be into tho, like I think there's only breast growth right now but nothing else, no dicks, butts, hips, lips, or whatever else.