68mm x 113mm BB, that's what's normally used for triples on road style, which is only part of the equation and has absolutely no correlation with crank length.

UN54 is a shimano cartridge JIS square taper BB, so you can use sugino, older shimano or there abouts.

http://www.sheldonbrown.com/bbsize.html

I'd just take it to the shop you bought it from and ask them if they will swap out the cranks for shorter ones for you.