XD Why not just make them human-sized? Like. . They look completely normal but, yeah, they're not.
I used to have all my canon characters completely OOC, but you just gotta stop and think "what would this character say/do right now" and imagine this happening in the anime. That's how I keep them in character. It takes up more time, but it's really worth it.
I've figured since the psychics love their fighting, their bodies are properly built for it compared to the rest of the human race. Although that doesn't explain why they're so tall...
The major downside isn't on the person using it as much as the person receiving it. Since it's so strong, that's going to HURT, if not put them into comas. Although I'm sure that kind of power would be exhausting to gather up.
I don't know... reading my old fanfic, I found I wasn't that great on grasping how the canon characters would act. So I still worry about it. XD; These days, I'm more worried about people sounding the same. Some characters, I have their speech patterns down, and then everyone else is kind of in this blob...
Well, unless they have a high threshold for pain, I suppose they would get frequent concussions. XD But, man, it's your story. My character sometimes gets thrown into the ocean and she can't swim, so you'd think she'd drown. Crap. . I lost my train of thought. XD
If that's the case, they should have nasty side-effects. Like if your character uses Psybeam, they get really dizzy and can only use it once or they'll pass out. Something like that would work. The problem isn't giving them too many good quality, it's to give the good qualities their downsides.
I'm not really too worried about going OOC, especially considering I have a good sense of how the OCs would act in certain situations. And it makes it easier in Pokemon ('scuse the absent accent above the e) since everyone has a 'human' personality.
My psychics get slammed into walls, ceilings, streets, and whatever other random big things that are around all the time. In fact, that kind of tactic is the only thing that -- in traditional matches -- isn't a limited move. Which I wonder about, because if they get slammed too hard, wouldn't they constantly be getting concussions?
I always keep meaning to slip some psybeams in. I had them in my fanfic, but then I took them out since I thought they'd be too powerful. Unless I use it now BECAUSE it's too powerful... then again I'll be having fun in the future, editing all these fight scenes.
I think the biggest reason I don't write fanfic these days, other than the lack of ideas, is the fear of going OOC.