The tighter the bend radius, the easier it is to design around IMO, however it depends a little on what you intend to make. Sometimes I could see it nice being able to make the same bend radius with a 1.5, 1.75 and 2 inch tube. That said, I'm pretty sure both my 2" and 1.75" inch dies are 6 inch.
The dies will have their minimum wall thickness typically determined by the bend radius. For example, a 2" die with a 6 inch bend radius must be used with .120" wall tubing, whereas the 7" die works with .095" as well IIRC.