Inspired by the size class for items blog post the idea itself is fairly simple:

Weapons
-Larger sizes increases damage at the cost of accuracy and small weapons are much more accurate but less damaging.
-Larger weapons have a slower attack speed, small weapons faster.
-Using weapons of the wrong size nets an accuracy penalty.

Shields
-Oversized shields grant more DV, but have a greater accuracy penalty if its not your size. This presumes that large creatures naturally have lower DVs(which the shield will make up for), but small creatures can use oversized shields for DV at the cost of accuracy.

Armor
-Larger armors are biased towards higher PV and smaller towards DV.
-Mis-sized armor nets a speed, accuracy and DV penalty based on the size gap.