The most common size of Anglo-Saxon shield appears to have been 50-60cm, which is rather more than buckler-sized. There were buckler-sized shields too, about 25-32cm in diameter, but these were less common. There were also large shields, about 90cm across, but again less common. See table 12 (pg 46)and figure 34 (pg 47) in Tania Dickinson and Heinrich Härke, Early Anglo-Saxon Shields, Society of Antiquaries (1992) for a distribution of shield sizes.

50-60cm is smaller than, e.g., typical Viking shields of 80-90cm. A medium-sized shield of about 60cm will offer less cover against arrows than a large shield (90cm). Medium and small (e.g., bucklers) shields are effective against single hand-to-hand weapons. Bucklers were in use long before the Renaissance - the fightbook I.33, from about 1300, is for sword and buckler.

Compare a 60cm shield with a 90cm shield. For the same thickness, the 60cm shield is about half the weight, and is correspondingly more mobile. It only needs to be moved 15cm further in any direction than the 90cm shield in order to stop an attack. In addition, it blocks the wielder's own vision less, and offers the opponent less leverage to manipulate it. Against a single opponent with a hand-to-hand weapon, a medium or small shield can be better than a large shield (see I.33). It is also easier to carry.

Where a large shield is better is against arrows, and perhaps in the shield wall or phalanx, where one is facing many opponents and the spearhead that you aren't watching is dangerous.

A medium-sized shield provides a reasonable compromise between the mobility of the small shield and cover against arrows of a large shield.