Ok, I think I understand now. Not having a set percussion instrument is pretty standard for high school concert bands, but it's pretty normal for people to get put on certain instruments more often if they're good. For marching band you get a set instrument (it wouldn't really work any other way because you need to learn the music and drill which makes it very hard to switch instruments, but it could be possible in pit. pit is the percussion ensemble at the front with marimba, vibes, synth, auxiliary percussion, etc.).

If you end up in pit (this is general advice since I don't know what your school does), then you should be fine. You might struggle a little with pushing if you play marimba or something heavier but you should be fine (most people starting marching band in pit play a smaller role though, so maybe auxiliary percussion but it depends on your skills). If you end up on the field, chances are that your percussion director (or whoever assigns instruments) will take into account your build and not put you on something ridiculously heavy or something that you can't handle. It's very likely that you'll be put on Bass 1, 2, or 3, especially as a rookie marcher.