There's two measurements for this particular belt type, being that it's a V-belt.
I assume, being that it's a deck belt, it to be 5/8" wide (thou it could be a 1/2" but not as likely).
The length is a bit trickier.
If you can lay it out on a flat surface and mark a spot both on the belt and the floor, then "roll it out" until you get all around, then measure the distance.
That or if the belt is shot, cut it and lay it out flat, then measure end to end.
That should give you the length.
Again more than likely it's a 5/8" which I believe makes it a class B.
Keep in mind your belt will need to fit Industrial / Agricultural use specifications.
Now since you were so nice as to post all the numbers I'll go see what I can dig up, but don't hold me to it.