Do larger sheets handle more wattage? 1/4 of a larger sheet 1/4 the watts?

Basically, yes, exactly. More pedantically, if sheet B is twice as wide and twice as long as sheet A, sheet B will have

**almost** four times the current handling capacity of sheet A (for a given temperature rise). Why not exactly four times? Because the edge of the sheet has slightly better ventilation that the center areas, which are surrounded by hot metal on all sides. The larger sheet has a smaller perimeter-to-area ration, so not quite as good at rejecting heat per unit area.