Get your model number and put it into a Google search like this "snapper model *******" You should get a few companies that list the parts for it.
Z-180 3k
A102
Question is too vague as ther are more than one size deck.
what size is the belt for a 46 in cut deck belt with 6 pulley's.
What is a size of ared snapper fish
43
Most LT12H mowers take a #213676 OEM belt or #6885 Gates belt for the blade application
Hard to answer without a deck size.
Gates
5/8 x 57"
a115 for triple deck
somewhere in the 44" size but best to look on the belt for a number or remove it and match it to a new belt. might also measure around around the pulleys with a small rope/string if you don't have a old belt to get the size of the belt.