I have a Murray 42" riding mower and can not find the size of the secondary drive belt. Also, I do not have the owner's manual.

Hi,
Without model number information, it is indefinite. The number would be under the seat or on the rear of the frame. As a rule a 42" machine has a drive belt and a deck belt.
Ilooked up this representative model at:
Murray Lawn mower Parts, Murray riding mower and Murray push mower parts
with an example model number and came up with 37X87MA for drive and 37X88MA for deck.
Going to a across site by the same company:
MURRAY Belts Listing
I came up with 1/2" x 97.5" and 1/2" x 89.1" respectively. Originals are recommended, especially for the deck as it is usually a wedge instead of a V.
Parts breakdown in pdf format also attached.
Peace,
Clarke