I'm having a really hard time getting a grasp on serving sizes. I understand party cakes vs wedding cakes, and Wilton is different than Earlene's, etc., but I can't seem to come up with a solid answer when someone asks "so what size cake do I need for this many people".

Here's my specific issue...I'm making a wishing well cake for 15-20 people, with a filling. Do I make two 8 inch rounds and slice those horizontally (I think the term is torte?), essentially having 4 stacked thin layers? Is that enough to serve 15-20 party cake slices? Do I go bigger?

Thanks for any help!

That will be enough cake when you take into consideration the filling and icing.  The cake should be on the taller side.  An 8" cake could feed more than 20 when torted.