2000 Subaru Outback self tensioning devise & drive belt

The bearings in my self tensioning disc/pulley/devise siezed and as a result my drive belt snapped. I've now got two problems / questions. The new drive belt supplied is about 10cm longer than the original one (I'm sure there are no bits missing from the old one). The dealer has checked the vin no & is sure they have given me the right belt. Is this a problem? I've knocked out the siezed bearings from the old disc & have asked for this to be repaired, they have told me that I have to purchase a whole new pulley and they can't replace the bearings & it has to be done by a Subaru mechanic. How do I replace the drive belt and the bearings in the self tensioning devise. Plus will this automatically correct if the belt is loose once it's fitted?