[1.6diesel]

I just recently bought a 91 Jetta with a 1.6 L diesel engine. it needed a few things so i'm fixing it. replaced some injector return lines, but a battery in it, and a Altenator. Now there is my problem. The altenator belt is shot so i bought a new one. the one i got was too big. So I took the old belt in and they matched it up. the second one i got looks really close but is too tight, too small wont go over the altenator pulley even with the idler out of the way.
Any one have any idea whats going on or what i'm doing wrong? is there a special belt? any help would be great.
thank you
Brian