I think the horizontal oval is 3.5" and vertical is 2".
I' m not planning on headers, there is an adapter from round to oval.
ROUND Tubing / OVAL Tubing Sizes
5" Rd = 3-1/4" x 6" 5" Oval
4" Rd = 2-3/4" x 4-5/8" 4"Oval
3-1/2" Rd = 2-1/2" x 4" 3-1/2" Oval
3" Rd = 2-1/4" x 3-1/2" 3"Oval
2-1/2" Rd = 1-3/4" x 3" 2-1/2"Oval
I've got some 2.5" oval tubing for a project I'm building....