I have a Sears Suburban 12 917.25550 and the belt from the engine to the transmission is missing. Can someone please let me know what size this belt is?

Well I have the same tractor and when my belt brakes i just go to the closest napa store and tell them what you need a belt for whitch is the sears and they should be able to get you one!!

Sears part # 2295R which is a 5/8" X 82" belt.
I find that a Gates belt size B-80 works best on my Sears tractor.
All your parts can be found at www.searspartsdirect.com.
Also, on this website is a belt conversion chart, which is very helpful.