Box tubing of 40 x 10 x 1.5 mm is a good choice. It has the same width as the bottom bracket and is narrow enough to fit in the small space between the tire and the chainwheel.
Easier to get is 30 x 10 x 1.5 mm tubing. This is strong enough, but does not make the frame stiff enough. So additional strengthening needs to be made to make it more stiff, which will make the frame more complex to built.
So in short if you want to make a P4 type frame, then you are certainly better of with 40x10mm.