What shield size should I use?

I'm not familiar with the mechanics of AD&D at all and haven't played many games like this before (I played Dragon Age: Origins). I tried looking on the web but there was very little information.

I'm a Chevalier (Paladin/Fighter) with proficiency in long sword and 1 sword / 1 shield style. Playing Baldur's Gate 1 in BGT.

There are 3 size of shields, and I don't know what the advantages and disadvantages of each are?

IIRC, Baldur's Gate used fairly accurate 2nd Edition AD&D Rules.

In 2nd Edition AD&D, the size of the shield translated into the number of attacks per round that the shield provided an armor bonus against.

E.g. a small shield would provide a +1 bonus (+/- any magical modifiers) to only the first attack in that round. A medium shield could be used to block against more attacks per round than a small, and a large more than a medium.

The downsides of larger shields are weight restrictions (the bigger they are, the heavier), and class restrictions (some classes are restricted to small shields.

edit: I could find no reference to indicate whether BG implemented this rule. However, this site shows that larger shields provide additional bonuses against missle attacks.

Armor/Shield    Armor Class                                     Usable By
Buckler         -1, no protection from piercing and missiles    F/R/P/Th/B/C/Dr
Small shield    -1, no protection from missiles                 F/R/P/C
Medium shield   -1, protection from missiles                    F/R/P/C
Large shield    -1, –2 protection from missiles                 F/R/P/C

If you're outfitting a warrior class party member, you'll want to focus more on the abilities the shield proffers, rather then the size of the shield.

For example, a Dragon Scale shield would offer some resistance against a particular elemental damage type, the Shield of Harmony would protect you against Charm, Hold, and Confusion.

As far as I know, the only difference between a small and a large shield is the bonus against missile attacks, bar the obvious difference of AC level.