What is the size of the belt required for a 46 in 20 hp deck on a murray ultra lawn mower

Hi,
It is always wise to give a model number here (from the tag under the seat.) That said, a common configuration is to have a Murray 37X66 (1/2" x 77") and a 37X89 (1/2" x 87") belt as the primary and secondary belts.
Not hard and fast as some others come into use, also.
Model numbers count! And, I can get you parts breakdowns!
Peace,
Clarke