The Effectiveness of Shields and Shield spec:
A) Shield spec has a base 5% chance to block +0.5% per spec point
B) Enchanted bonus on Shields also affects chance to block (Unsure in what way, maybe A+B, or A+(AxB))
Small shields: (supposedly) can block only 1 target, and has the base chance to block
Medium shields: (supposedly) can block up to 2 targets, and has an extra 5% chance to block vs. Small
Large shields: (supposedly) can block up to 3 targets, and has an extra 5% chance to block vs. medium and 10% vs. small
Shield Sizes:
Round = Small
Kite/Heater = Medium
Tower = Large
Never heard of bonus % chance to block with shield sizes.
As far as I remember, it was based on testing back in the old days, could be different on freeshards today, depending on what formulas they have implemented.