Technically, I believe that these decks use a 3/8" belt. That being said, I have always used 1/2" belts with no problems.
The model 44 deck ( larger center pulley) uses the larger 81" belt.
The model 44A deck ( smaller or 'speed up' center pulley) uses the smaller 78" belt.
In addition to what fins said, I try to make sure that the portions of the belts that go up to the PTO are as close to vertical and parallel to each other as possible.
I actually use a 1/2" x 80" belt with my model 44 deck with great results.
