gas funnel dimensions

Reading in the book, "the rocket mass heater, builder's guide" by Erica and Ernie Wisner, in the 6 chapter, they write the "equivalente CSA for gas flows. I would like to know if anybody has built a bench for a 200mm heater with a diferent dimension than 200X200mm for the gas tunnel, for example 160X250 or similar. would this give any problem with the chimney flow?
Thank you.

Greetings Alvaro and welcome to the rocket mass heater forum.

Off the cuff, I would say that it shouldn't be a problem since your sticking with the same CSA, just a different shape. However, at some point a narrower channel would put a lot more friction on the flowing gases due to the boundary layer effect. A circle is the best choice as it puts the least amount of friction due to the lowest surface area exposed to the gases. A bell is even better as it imparts next to no friction.

Thank you Gerry. My problem is the lack of space for doing the bench. A bell will take more space. I am trying to do a bench of about 60cm wide for 47cm tall for a BBR of 200mm. This is the reason because I try to find anybody with the experience of have done a similar bench.

A bells shape and size are very flexible to be large or small, tall or squat, depending upon your space and goals for heat extraction.
A good description with pictures can be seen here: bell theory
I understand if that's not the way you want to go. Just wanted to make sure you weren't limiting yourself to a system that you felt confined to stick with.  

thanks a lot Gerry, I will think about it.