[DanEC]

On rectangular you have to go quite a bit bigger than that I believe. The cross section of a 2-1/2 inch pipe is 4.9 sq in. So you would need a 1 x 5 to get equivalent area. But to get equal flow I think you would need to go even bigger since the boundary circumference on a 2-1/2 inch pipe is 7.85 inches and a 1 x 5 rectangular pipe would be 12 inches - creating a lot of boundary drag and restriction in the rectangular pipe. Someone who remembers their flow mechanics courses better than me would need to figure out an equivalent size. Oval piping is probably a better approach where ground clearance is needed. .