Cover a large baking sheet with parchment paper. Trace bottom of a 9- x 5-inch loafpan on paper twice. Turn paper over; secure with masking tape.
Beat egg whites and cream of tartar at high speed with an electric mixer until foamy. Add sugar, 1 tablespoon at a time, beating until stiff peaks form and sugar dissolves. Using a small spatula, spread half of meringue 1 inch thick on each drawn rectangle to completely fill rectangles. Bake at 250° for 1 hour and 15 minutes. Turn oven off, and let meringues stand in closed oven overnight.
Tip: Trace rectangles on side of parchment paper that curls up. When you turn the paper over, it will lie flat on the baking sheet without having to secure it with tape.