Sizing and shape of the boiler?

[DAD300]

I find a lot of great theory about column size and height, but...

What would the geometric "perfect" shape be for a boiler, regardless of size?

Are there problems/inefficiencies in putting a small charge in a large boiler?

At this point I'm not worried about heating inefficiencies, just product quality/quantity.

I understand that increased surface area should enhance or speed take off, but assume there are diminishing returns at some point, as in column diameter.

I have an 8 gallon gas fired milk can and have just tried it with a 2 gallon charge. I believe I got a lower percentage/quantity return than from a fuller charge.

Great forum, thanks...