I would definitely not go larger than a 20# Propane tank. While a Ribbon Burner is a little more involved to build you will have much better heat distribution throughout the forge, better control of the atmosphere inside the forge, better temperature control and get better efficiency. The Ribbon Burner shown on my web-site does require a blower and electricity.
Frosty has designed an atmospheric Ribbon Burner but because it does not have the blower, and thus not have the air/fuel control, it may not have the advantages of a blown burner except that it will have better heat distribution and I suspect better efficiency than a typical atmospheric burner.
Let me know if I can help you.
Wayne