I've been asked to do a wedding cake to serve 300 people. I've only done a few wedding cakes, as I'm somewhat of an amateur decorator, and none have been this big. What size round cake pans should I use to accomodate 300 servings?

here is a chart to help you figure the pan sizes:
http://www.wilton.com/cakes/making-cakes/baking-wedding-cake-2-inch-pans.cfm

I had looked at that, but it didn't have the serving sizes for an 18" pan. The fewer tiers, the better, so I was thinking of using the 18" pan and going from there.

Oooh, a big one! I've done a few 6-8-10-12-14-16 (295 close enough) Very impressive size!

You can use this website to help you calculate

http://shinymetalobjects.net/cake/calculator/cakulator.cgi

I did a wedding for 350 this fall...... she didn't want a large cake, so we did a three tier ...12' 10', and 8''.......... I made them a SURPRISE grooms cake,( a cruise ship) but the majority was decorated kitchen cakes.... it was SOOO easy to cut and serve. They also got to have as many flavors as they wanted...... Just a thought.....

Sizes like this make me want to double barrel tiers and half tall tiers though too. This much cake gives you a lot of possibilities.

You might also want to think about transport of such a large cake....... even if you set it up at the venue, that torted 18" cake is going to be HEAVY.......

Because you don't do thing normally, save your poor arms and your nerves and do a normal size cake with sheetcakes as backup.

Thank you so much for your feedback. I've been so stressed about this cake, I hadn't even thought of doing sheet cakes. That is definitely what I'm going to do!!