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The sizes are referring to the blade width-edge to spine. 5/8 and 6/8 inches are the most common production sizes and should be fine for most people.

This is a link explains some razor basics...

I would suggest looking at Straight Razor Designs. You can easily find a razor, brush, and strop within your budget(plus cream/soap, aftershave lotion or balm). They carry Dovo, Boker, and Aust...