Use a stainless steel or glass bowl. Plastic bowls can retain a film of grease. Bowl size (and shape) matters. For proper aeration, a small mixer bowl is best for up to 3 egg whites; a large mixer bowl for 4 or more egg whites. When beaten, egg whites increase as much as 6 to 8 times in volume. The bowl should be large enough to hold the expanding whites, but not so large that the whites are spread too thin. The bowl should be deep enough for the beaters to make contact with as much of the whites as possible.