Ski hip protector padding typically ranges from about 5mm to 25mm, but thicker doesn't automatically mean safer. The material matters just as much as the measurement. Modern smart foams like D3O stay thin and flexible while you're skiing but harden instantly on impact, so a slim 8-10mm pad with this technology can protect you as well as a bulky 20mm traditional EVA foam pad.
If you want low-profile protection that stays hidden under fitted ski pants, look for pads in the 8-12mm range made with viscoelastic (impact-responsive) foam. They'll move with you on the slopes and won't draw attention in the lodge. For maximum protection—especially if you're recovering from an injury or have osteoporosis—pads in the 15-25mm range offer the most coverage, though they'll be more visible and may slightly restrict movement.
The biggest mistake is buying pads so thick and bulky that they're uncomfortable under your ski pants, because then you simply won't wear them. The best thickness is the one you'll actually keep on every run.