Magazine size or magazine capacity refers to the number of rounds that can functionally fit into a given weapon's magazine. The highest magazine capacity in Goldeneye Wii goes to the Vargen FH-7 with a minimum capacity of 50 rounds; the smallest non-special capacity (Special weapons: Golden Gun, MJR-409, etc.) is tied between 3 weapons, The Toros AV-400, The Walther WA2000, and the Gambit CP-208. All have minimum capacities of 5 rounds.
Magazine capacities are usually standard for weapon types, with most shotguns holding 8 rounds, most assault rifles holding 30, and sniper rifles holding either 5 or 10 rounds. However, this does not hold true for submachine guns; their capacities range from as little as 20 rounds, to as large as 50 rounds per magazine.
Extended magazines also throw another wrench into the gears, as they take a percentage of the weapon's minimum capacity (~15%) and include it in the weapon's loaded capacity; for example, the already high-capacity Vargen receives 13 extra loaded rounds, bringing its loaded capacity to a monumental 63. Unfortunately, reserve capacity is not affected, which means you may find yourself burning through ammunition a little more quickly than you mean to. For example, the quick firing Strata SV400 will scorch through its 38 round extended magazines in approximately 2 seconds, and with a little more than 5 magazines in reserve, this can spell "Pistol time" very quickly.