In large bowl, combine sugars, margarine, peanut butter, vanilla and egg; beat well.
Lightly spoon flour into measuring cup and level off.
Add flour, baking powder, baking soda and salt; mix well.
If you are making the large cookies, it helps to chill the dough for an hour or so at this point, before shaping. It seems to make a firmer cookie--less likely to break. If you aren't concerned about that, you can skip this step.
Shape about 1/4 cup dough (for 3-inch cookie) into a ball and place 4 inches apart on ungreased cookie sheet (6 per sheet).
Bake at 375° for 13-16 minutes.
Remove from cookie sheet and cool completely on wire rack.