I am making a topsy turvy cake for 70 people.  It will be a two-tiered cake, tapered and slanted.  I have no idea how big to make it.  I was thinking possibly a 12" cake on the bottom tapered to 10", and a 10" cake for the top tier tapered to 8".  Will that be enough?  I am thinking of making each tier approximately 4 layers with filling.  

Topsy turvy or not a cake will still be sliced the same.  https://www.wilton.com/wedding-cake-guide/cms-wedding-cake-data.html

Ugh, posted before I was done!

I think what you are planning will be more than enough for the amount of guests.

cake servings go by footprint -- so you'll have a 10 x 8 which is 62 according to the chart in the link freckles posted for round cakes -- if you think the cake server will be efficient enough to get the extra servings out of the overhang -- you're there but that's pretty unlikely-- if you wanna play it safe do three tiers  at least--

a one inch difference on each side is not that noticeable -- it's a pretty taper but not much for a whimsical cake

70 is a lot to serve for a topsy turvy with two tiers -- maybe do squares -- but I think you are gonna need to up your game all the way around