I am going to make a cupcake stand to hold 50 cupcakes and a 6" cake on top. Can anyone tell me what sizes I need to make the rounds for each tier. I figure either a three or four tier tower. Thanks

I would guess four tiers.
The link below is three tiers for 24 cupcakes, but it gives dimension on each plate. so maybe that can help a little.

http://www.shopbakersnook.com/m5/8167--black-cupcake-stand-that-holds-24-cupcakes.html