Generally, with DOM links.. thicker walls do more to resist denting.. larger diameter does more to resist bending. Of course, thick walls and large diameter also equal more weight and higher cost. So, we try to find a good balance of durability, weight, and cost with our links.. especially the lower ones (subject to more abusive contact with obstacles).
On my old buggy, my lower links were 2"x 3/8" wall DOM. I bent 3 of them (slightly) the first time I took the rig out. Granted, this was a 5000lb buggy driven with no regard for drivetrain longevity.. I abused it pretty good and often used the lower links almost like "ramps" for getting up on obstacles..
If it were my rig... I would probably opt for at least 2" diameter lower links.. with at least .250" wall thickness... but preferably 3/8". If I planned to drive the rig aggressively/abusively, I'd probaly step up to 2.5" diameter and 3/8" or even 1/2" wall thickness. Heavy but tough.
In the end, only you know how you plan to wheel the rig so I'd plan on lowers that will stand up to your style of driving.. with a bit of "overkill" added just in case.