Mounting brackets, around 6mm, use some 6mm angle as well to bolt up to the front crossmember and mounting plates. Try to hook into this doubled up section for the recovery hooks as it will be 12mm thick and pulling straight down the 'frame'.
For the tube, go with whatever looks good or suits your design for diameter, maybe grab a few offcuts to help visualize it. Do you have a bender for tube, tube bars can look great but nly if they have nice bends
Cheers
Steve