I need to make a birthday cake for a teen to serve 75 people. I just need some advice on sizes to use to make one cake, or possibly tiered.

Girl, you have come to the right place! There's a great little cake calculator that was posted on here by a CC member named metria. Here's the link:
http://shinymetalobjects.net/cake/calculator/cake_calculator.cgi
You can calculate the number of servings a cake will give you either by volume or from the Wilton cake charts on this site. You can also find out how much batter and icing you'll need, so it's a very helpful tool. I use it all the time and cannot thank metria enough for coming up with it.

http://www.wilton.com/cakes/making-cakes/baking-wedding-cake-2-inch-pans.cfm

WOW! Thanks Marianna46 (and metria). That is going to come in very handy. No longer will I have to worry about my inability to do simple math in my head or the laziness that keeps me from looking for scratch paper LOL

THANKS so much!!! This is great, I'll be using it all the time.