Minimum weight on A-class is for the boat and full rig only: 165 pounds = 75 kilos. See the box rule at www.usaca.info
We have tall skinny skippers, some short skinny people, tall large people, short large people.... but overall, a lot of 165-175 pound people seem to do very well. Except for Lars Guck, who is a substantial guy and killer downwind, and Pease Glaser who is rather taller than I, and killer downwind too. (personal observation, Islamorada, Florida, January 2006). So what do they know about boatspeed that I don't know? Welll... a lot to start with!