Most online stores make you specify A/C or no A/C and power steering versus no power steering. Most belts have part numbers that describe the length of the belt in inches and number of ribs.
Here's a screenshot from RockAuto:
View attachment 116330
So for example the Dayco belt for no AC and no PS is 5060695, which tells you it is a 6 rib belt with 69.5" of total length. The 5060715 (no AC, with PS) is 6 ribbed with 71.5" of total length.