How do I figure out how many and what size tiers to use for a cake to feed 70 people?
Which combinations look the best?
i'm a newbie at this! Thanks!

If you go here, it has some basic guidelines to follow and will give you the serving amounts per cake size. Keep in mind that you might want to adjust the serving sizes based on personal preference and the event it's being held for.

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

As for which shapes to use, that depends on the look you are going for. Round tears are always the easiest to ice though because you dont' have to worry about getting straight edges (like on square corners). Hearts and ovals would be easy too.

I like a 12-9-6 for that number of servings (it's 80 by earlene's chart).

http://www.earlenescakes.com/ckserchart.htm

Different shape combinations:
http://www.wilton.com/cakes/tiered-cakes/serving-amounts-under-100.cfm

I would do 6/8/10 round, assuming using all 3 tiers. That's 78 servings. For cakes you sell it's best to use the Wilton charts. Why?
1) You will make more money.
2) You will have plenty of cake (look for IndyDeb's cutting chart and pictures.)
3) The caterer will cut to the Wilton charts anyway - they all do. If you use Earlene's charts you're just giving away cake.

Everything leahs said is completely true. I should have clarified that a 3-4" difference between tiers is more aesthetically pleasing to me, and I just plain like the look of the 12-9-6 combination. Whatever sizes and serving chart you choose to use is perfectly all right, that is just my own opinion.

And what Kelley said is why I like to have the odd sized pans--could not live without them. I think an 11x8x5 round is what I might come up with. That's just over 70--but under 80. I think that's about as small as it can go & be rounds.

I do a lot of 6/9/12/15 wedding cakes. Since I give them a gift certificate for a fresh anniversary cake, that serves 185. I totally like odd sized pans!

but whatever combination you use, you charge them for the number of servings it's DESIGNED to serve (preferably by the wilton chart) ... not by the number of pieces they plan to cut from it. If they want cake for 70, and the pan combo you have to use adds up to 79 or 82 servings, then you charge them for 79 or 82 servings.

This is not scientific so nobody shoot me but I take the difference between the two even sized pans. For example the difference between the 10" & the 12" is 56-39= 17 So I take half of 17 which is about 8 and I add that to the 39 and get 47 servings for the 11".

What I go by is:

square
7" 25
9" 41
11" 61
13" 85
15" 113

rounds
7" 18
9" 32
11" 47
13" 66
15" 88
17" 113

Maybe 8 servings out of a 5" --not too many servings out of a 4"

That's how I do it. Pretty sure my math is right-never had any issues but I'm always calculating and re-calculating, so feel free to check and re-check.

What K8 said. I do a schematic/cutting chart on the computer.