What is the relationship between the distance/area between the base of a fire in a fire place and entrance to the chimney, and the diameter of the chimney?
I don't think there are any hard and fast formulas. If you have an existing fireplace with 14" 16", 18" fire back, the basket that fits in should give you the right level for the fire. If it differs from this it can depend on all sorts of factors such as the height and width of the chimney and even if there are any trees around. In that case you probably need an expert to come and look at it.