Round is fine but I definitely prefer to use plate for my firebox. You can work all the dimensions so that it sits properly, allows length for your logs and is sized properly. The square box is also easier to work with...such things as air vents, hinges, doors and log racks are always easier to fit to a flat surface.
Yes, size is important. I believe the current recommendation is to make firebox 110%....somebody correct me if I’m off.