No he's asking which kind of shield is best: Large shields with high block strength but low block rate, or smaller shields which have the opposite properties.
So far i've only been using large shields. I suspect, as with most things, that this is a situational issue. Small shields will be better for low damage, fast attacking mobs, such as pugilists/monks. Larger shields are probably better for slower, heavier attacks. Or i could be totally wrong and it might be designed to all even out and just be a personal preference thing. Needs more info.