What are the bonuses or penalties associated with shield size in AD&D 2nd edition? There is some disagreement on this in a Gaming.SE answer; this seems like a much better venue for posing the question.
There are four shield types in Second Edition - body, buckler, medium, and small. The benefits and penalties are clearly spelled out in the rules.
Shields: All shields improve a character's Armor Class by 1 or more against a specified number of attacks. A shield is useful only to protect the front and flanks of the user. Attacks from the rear or rear flanks cannot be blocked by a shield (exception: a shield slung across the back does help defend against rear attacks). The reference to the size of the shield is relative to the size of the character. Thus, a human's small shield would have all the effects of a medium shield when used by a gnome.
A buckler (or target) is a very small shield that fastens on the forearm. It can be worn by crossbowmen and archers with no hindrance. Its small size enables it to protect against only one attack per melee round (of the user's choice), improving the character's Armor Class by 1 against that attack.
A small shield is carried on the forearm and gripped with the hand. Its light weight permits the user to carry other items in that hand (although he cannot use weapons). It can be used to protect against two frontal attacks of the user's choice.
The medium shield is carried in the same manner as the small shield. Its weight prevents the character from using his shield hand for other purposes. With a medium shield, a character can protect against any frontal or flank attacks.
The body shield is a massive shield reaching nearly from chin to toe. It must be firmly fastened to the forearm and the shield hand must grip it at all times. It provides a great deal of protection, improving the Armor Class of the character by 1 against melee attacks and by 2 against missile attacks, for attacks from the front or front flank sides. It is very heavy; the DM may wish to use the optional encumbrance system if he allows this shield.
The shield becomes much more valuable in defense if, in your campaign, you use "Skills and Powers" or "Combat and Tactics" rules, and allow the character to take shield profeciency. That table has additive defense for each of the four shield types, increasing with the size of the shield. That is additive the normal amount of defense obtained from shield use. These really makes the shield more desirable to use. I had a campaign where most fighters were using two weapons, now most opt for the shield.