I have never done a round cake to serve this many people before.
I have done a 2 tiered cake to serve 20-30, but not 30-40.
What size would you do on the bottom? Would a 2 layer 8 on the bottom and two layer 6 on top serve 30-40? How many would a 9-6 serve? I have tried consulting the serving chart on Wilton's site, but I find that to be pretty inaccurate whenever I have a party for some reason.

either a 1/2 sheet or a 10" round. if you want to do a tiered cake a 8" and a 6" would be fine

Go to earlenescakes.com. Her pieces are a nice size. Bigger than Wilton.

I always use a 6 and 10 for forty people.

It sounds like this cake is for your family; if not then you need to base your serving on the industry standard of 1x2x4.
An 8x4 round will yield 25 servings; a 6x4 should give you 15 so that is your 30.
If you feel extra generous or the customer wants to *pay* for extra then a 9 or 10x4 w/a 6" would be the next choice.

Just be sure you're not cutting the cake into the usual pie-shaped wedges, but are cutting them in rectangles. Here's a step-by-step on how to cut round (and square) cakes: http://cateritsimple.com/_wsn/page10.html

6/8 cakes serve 12/24 (total 36) per the wilton chart. A 6/10 serves 12/38 (total 50) per the wilton chart.

I cut cakes this size at all family events and it works out fine, especially if it's a holiday dinner where there's lots of other food. A 1x2x4 is bigger than people think it is. They hear "one inch" and think "paper thin" ... it's not.