Hi! I need to make a 3 tiered baby shower cake for 85 people, and she wants each tier to be 3 layers. In your experienced opinions, how big should I make each tier? Any input would be greatly appreciated, thanks!

Three tiers makes some pretty big slices. I end up cutting even smaller pieces with three tiers so a 6/8/10 should offer up plenty of cake.

Sorry, I meant layers, not tiers.

Depending on how the design would work I would try a normal double layer 6", a triple layer 8" and normal double layer 10". Thats a big baby shower!
Or you could always do what ever size you want and have an extra cake if you need more servings. Like maybe some baby blocks or cupcakes. I would focus on the design then see how changing the sizes of the cakes will work with it.

Does she mean three 2" tall layers for a 6" tall cake, or she doesn't care how tall the layers are as long as there's three of them?

If she wants 6" tall tiers (and it doesn't matter how many layers it takes to achieve it), you could bake six 1" layers per tier and separate them with a cake board in the middle at 3" to cut servings that are 3" tall apiece. Using a 6x7x8 would yield 83 servings (18/29/36). HTH.

Round or square? I've talked several people into adding a square to the bottom tier if they're worried about not having enough cake.

Yes, as SpecialtyCakesbyKelli said, it depends on the shape of the cake.
A full sheet will easily give you the needed servings.
A 12x16x4" sheet also will give you like 90
So will A 16x4 round
And so will a 14x2 sq.
The large oval 4" tall also provides the # you need

So you see it depends not only on the size but the shape also