The beginning of forge design is the study of burners. The proper end point of burner design is the equipment they power. Lumped together, these subjects are too unwieldy; at the same time, they can't stand-alone, since their study is as intertwined as the equipment itself.
What kind of forge is best is an arbitrary question, without any adequate answer. But, whatever design you favor, there are common factors which lead to a desired end point--or away from it.
Size: Bigger is never