The part number for the Dayco Poly rib belts contain how long they are.
7PK1076 = 7 Ribs and 1076mm long
Normal 300tdi belts are: 7PK1580 or 7PK1575 , If you measure the distance that your new shorter belt should be and go to a motor factor, they should sort you out.
Catalogue:
Dayco Web...