When I ride trials I have a really hard time jumping while standing on my pedals. Instead I just stand on the cranks, but I’m worried this might damage my unicycle. Think crank length might be part of why it feels so unstable to stand on my pedals. My uni’s cranks are pretty small, 115mm. Should I get longer cranks? What should I do? 
I would suggest getting bigger ones
I have trouble on 125s so I got 137 KH moment cranks for my kh20 Great cranks and really good to stand on.
Maybe your pedals aren’t big enough?
Well, I hear that cranks in the 137mm range are optimum for trials, and street riders seem to prefer 125’s, or in that range, and freestyle are usally quite short. For trials and street though it’s pretty much personal preference, and your riding style, height, weight and all that can also be a factor.
137mm FTW
Yes, longer cranks would definitly help you.
Do you have a splined trials unicycle? If you do then you can get splined cranks as mentioned in this thread, but if not then you’ll need square taper cranks.
Standing on the cranks to hop is less likely to damage your unicycle than standing on the pedals, the effective distance that generates the moment in the axle is smaller. It was a somewhat commmon technique before strong unis were commonly available.