Would it be possible to lose 5 or 6 sizes by May?

I have signed up for kickboxing and am changing my diet and cutting out sodas, booze and fast foods and loading up on fruits, veggies, whole grains, whole wheat, lean meats, etc. I am currently a size 11/12 and weigh 167. Could I get down to 140 or 135 and be to a size 6/8 by May? Thanks!