As a good rule of thumb, about 65% of the invited guests will show to your wedding. So that would be about 120 people. With your first thoughts on the cake sizes she will have double the cake she will need. I would severely downsize the cake. Even 198 servings (like the sizes I said in my previous post) will be a lot more than she needs.
You could do a 14" round, 10" square, and 8-6-4 rounds to give you 160 servings (14" round 75 servings, 10" square 50 servings, 8" round 20 serv, 6" 10 serv, and 4" 5 serv). I use a modified Wilton chart (just rounded down to make nicer numbers for math) and I have never had a problem with not enough cake. I use the same chart for birthdays as well. Earlene's chart takes money out of your pocket and leaves too much cake. I used her chart for a couple cakes and quickly abandoned it.