I'm making a 2 tier sqaure wedding cake for 30 people.

What sizes should each cake be?

(Plan on extra guests possibly, so extra cake would be fine, so up to 40 guests?.)

It's a buttercream cake.

It depends on several things...how tall are you making each tier?  Do the bride and groom want to keep the top tier for the 1st anniversary? Who is cutting the cake and do they know how to professionally cut/serve it?

Standard wedding cakes have 4" high tiers and the servings are generally 1x2x4".  Therefore an 8" square cake at 4" high would give you approx 32 servings.  

If cut properly,  8" and 6" will give you more than enough cake and then you could always , if needed, do a little 4" for the bride and groom to freeze for their anniversary.

They weren't clear on if they were keeping the tier. It's a second wedding so I doubt it.

I have no idea as far as cutting it goes they are in charge of that or finding someone to do that as I won't be doing it.

I'm doing a checkered cake, so it each tier section will have at least 3 layers.

So your suggesting 4, 6, 8?

Would a 10, 8 be too big?

Read more at http://www.cakecentral.com/forum/t/827583/need-to-feed-approx-30-people-for-a-wedding#post_7646915#cuI3Ce43kUBlYXV2.99

See this chart for approximately how many people a 4" high cake will serve: Cake Baking and Serving Guide

It lists both party serving size and wedding size. A 10 inch square alone will serve 50 people (the wedding serving size 1x2x4")