anything less than 1"x.083 (then it better be short). As stated prior as length increases diameter should too. There is a customer car here in the shed that has a .875dia by 44" long that I am going to recommend a larger diameter. The bending moment forces increases dramatically as Tube Dia increases marginally so bigger in this case is better with little to no weight penalty.
The advantage of a wishbone is you can make suspension tune changes (4 link bar position/length) without adjusting the wishbone. Additionally the wishbone can be mounted above the axle center-line allowing for center section/drive shaft drop out ( 8 3/4 or 9") where a diagonal link cant. Also a wishbone has three load points vs two on a diagonal link.
An X link is to a degree an upgrade from a wishbone as it has four load points and it pivots about the center of motion... seems like over kill but they were introduced at a performance level above my experience so there may be some benefit .

Thank You Sir !
looks like i will be building a new diagonal link for my "eternal project". the diagonal link now is 7/8x1/8" tube, 34" long, including the 5/8" heims.
would 1 1/4"x1/8" wall be good for this ?
believe it or not, someone told me years ago when i was making the one i have now, to use 1/2"x.083 wall tube with 3/8" clevis ends. ..........er, um......NO.

Pretty much every suspended dragster on the planet uses them.