I want to buy a bike. I have a bike who may interest me, it is a Tri Bike 51cm. I was wondering if it was OK for me. I know that for my height 53-56 cm is better but I'm only 115 pounds. Would that be OK for me?
A 51cm frame sounds too small for you, assuming you're average limb length for your height.
You can put the saddle up really high, but that makes it a more aggressive body position, and your seat post is not as well embedded in the frame.
You don't mention your age - I'm guessing you may still be growing, in which case the too-small frame will be getting smaller as you grow.
Answer Pass on this bike - there will be another.
I am 5'10" and had the use of a 53cm while I decided what to buy. This was clearly too small - had some knee discomfort and felt "bunched up" on the bike. I bought a 58cm and can most definitely feel the difference. My arm may be a little longer than normal, but I am happy with the 58. Don't settle for one that is not the right fit.