The only coaster you may have a problem with is Colossus - because it's very pinchy in the seats and if you are tall it can add to it - as in there is naff all leg room - my nieces dad didn't fit in it and he is around 16/17stone (just to add he is very tall - 6ft 4ish which didn't help!) - but they have tester seats outside of all the coasters bar Flying Fish and X which have lap bars so you'll have no problems with those.
As for flat rides, Vortex and Samurai have tester seats (when they are working!), quantum has a lap bar, as does Rush. Zodiac has no restraints so again, no problems there. Detonator has over the shoulder restraints but I'm unsure if they have a tester seat as I've not ridden that thing since 2013.
Water rides you'll be absolutely fine on and pretty much everything else I'm sure - I've probably missed stuff out but it's late!
As Roodie said - don't be afraid to try the tester seats.
Have a great time and try not to be nervous about whether you will fit - enjoy the coasters and make sure you ride Derren Browns Ghost Train!
Han30's post in Can I use a college id for the student discount? was marked as the answer