Whatever size you'd like ... consider the height and depth of your shelving, how many folds you want to put in the fabric and the amount of fabric you are putting on it.
Fabric width is 40-45". So 1/2 as it would be on a regular bolt would be 23". If you you wanted a 2nd fold, then 12-1/2". And I'm sure that 12" would be more than adequate as we seldom see 45" fabric now.