With skill, one can make almost any size blade with a very minimal heat supply.
The real answer to your question is:
It has an 11" chamber. The first and last inch are probably not the best part for working, so it should heat about 9" of steel . Moving the blade in and out will even up the heating. For practical purposes, a small forge like the Atlas is good for blades with up to 6" edge length. HT gets trickier above that.