I want to do my first stacked cake. I have 9 inch, 6 inch and 4 inch rounds available. Although for a birthday, I think two levels would be best.

The person I'm making for was going to order a costco half sheet cake but I volunteered and I 'm not sure how to size this thing.

which pans and how many panfulls should I bake in order to serve at least as many as the costco size?

Did you ask how many you will need to serve?
Here is a serving chart.

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

A nine inch with a six inch on top would serve about 30 people according to this chart: http://www.baking911.com/cakes/numberofservings_guide.htm

It would serve more if you use the wilton serving guide, but I personally think those servings are on the small side.

Not knowing how many servings you need, makes it hard to answer. However, make sure each tier(level) is at least 2 layers, a single layer, even torted and filled, looks skimpy and weird to me.

Here is the wilton wedding chart that I use for all cakes: http://www.wilton.com/wedding/wedding-cakes/wedding-cake-data.cfm I use it to determine pricing. They are welcome to cut the cake any dang way they want, but they are paying for the servings according to this chart.

Here's pics of pieces of cake cut by the wilton chart: http://www.cakecentral.com/modules.php?name=gallery&file=displayimage&pid=1156785 Most people think "paper thin" when they hear "one inch". As you can see, it's not.

I tell clients its about the size of a peanut butter sandwich. If they cut their pieces of cake bigger than that, they need to order (and pay for!) more cake. I have found this size is PERFECT for a little kids birthday ... and every adult at every party I've ever been at has been fine with this size also.

Here's the method of cutting the cake to achieve those sizes: http://www.cateritsimple.com/id10.html Feel free to print this off and hand it out or refer clients to this page for cutting instructions.

Dont' give away free cake just because the client has a family that eats like Jethro Bodine. KFC doesn't do it .... I dont' do it.