Monogram Cake Topper...What Size?

Hey ladies...I want to order a monogram cake topper (single letter) but i have no idea what size to get. I haven't met with the rh to pick out the cake so I do not know how big it will be. I plan on having about 150 people at the wedding.

If you have any experience with these, please suggest what size letter I should get. 4 inch? 5 inch?

Thank you!!!