There probably isn't a really good answer to this question. As the post that Thor linked shows, there is no standardization in edger sizing, made all the more complicated by the fact that there are several different styles of edgers .. and even from the same maker no size standard between those! The only general thing that is true is the bigger the number, the more leather it removes .. within the same line from the same maker. I'm lucky enough to have found one, unbranded #3 that works for most of the leather that I like to work with, but it was just plain luck to have found the right size.
There is, however, at least one possible solution to the problem that is reasonably inexpensive. Jeremiah Watt offers a line of western edgers that are reasonably priced at $45US for a full set of five sizes. http://www.ranch2arena.com/hsbtqctools.html ... They are also available from weaver leather supply, although I did not see the full set on their website. They aren't pretty, with a plastic handle and interchangeable blades, but certainly affordable!
Nigel Armitage recently reviewed these favorably in a video on youtube. https://www.youtube.com/watch?v=liakL3TyPA0
These are probably going to be my next new tool purchase .. Sometime in the not-too-distant future. When I get 'em ... I'll report back. If anybody else has them already, let us know what you find!
Bill