I've been meaning to scan this for years, this guide shows the drive belts sizes for all of our favorite washers (both wringer and automatics), dryers and ironers.  It is from sometime in the mid to late 60's and goes all the way back to the 1930s.  I can't tell you how many times I've consulted this guide over the years.  Be sure to save it to your hard drive, I've scanned it at a high enough resolution that it will print out very clearly if you wish to print this 30 page book.

The link goes to a new program for upgraded members to download special little treats like this that I'm planning on sharing in the future.

The way you use this guide is simple, find your brand and model number and in the right hand column is the belt size.  Any belt that has starts with 2L, 3L, 4L or 5L is a standard hardware store belt.  The more common sizes such as the 3L's and 4L's are available at most Ace Hardware stores and the like.  Just right down the number and take and ask them if they have that size.  Ace almost always has every common belt I've needed over the years.  Grainger is another good source for belts and they have a much larger stock and selection.

I have had to fly out to Minnie time and time again to get my hands on this!