Wider, heavier shields have a better chance of resisting stuns, height has no effect.
Thanks Banks, this is the first complete description of block stun I've seen.
I've found that 3kg shields result in near nonstop block stuns, whereas a 3kg weapon (say Highland Claymore) won't be stunned except by heavy blunt weapons. The light shields already pay a severe penalty in durability and projectile coverage, so the susceptibility to block stuns is just insult to injury.
It's interesting that shield width has an effect. As if round shields weren't already the golden children of the shield line (since they are the only shields that seem to block projectiles from even slightly to the side), it turns out they are the best for mitigating block stun also. I'd like to see the kite/heater shields be given some kind of non-stat advantage over round shields (besides the mounted forcefield, which is stupid and should be removed anyway); increased reach on neutral and offensive nudges would make the most sense, given that the animation implies that the shield height affects reach (it doesn't).