What is the belt size for the mtd gt 18.5 46 inch riding lawn mower

Hi,
Well, if you want to do the rest of the work, like looking up which version of the 46" deck your machine has. I know of four that MTD uses. Here is where the cross reference is:
MTD Belts Listing (http://outdoorpowerinfo.com/belts/mtd_belts.asp)
Neither of the ones above are the length you gave, but you have to start with a model number to really know.
Peace,
Clarke