Vixjean- I don't think you are to short for the bikes. I am only barley 5' 1". I have to adjust the bike sit to the height of my hip bone. So, try to adjust the bike there and your legs should be at the correct length. The instructor should help you set up the bike.
Anyone that is thinking about the spin class. I do warn you in advance your butt will hurt because of the little bity seat. However, keep going to the class. After your third time of going to the class your butt will ease up.
Good luck.