I'm attempting to make a cake similar to this, but I don't know what size they are.

And when making these cakes am I making two of one size and two of the other?

I'm going to attempt to make everything from scratch. 

Please any information would be helpful

http://pinterest.com/pin/216946907021381065/

It's hard to really tell since there is nothing in the picture to suggest scale, but I'd say it's a 6" cake on top of an 8" cake.  Though I think it would look better if it were a 5" cake over an 8" cake... but that is just my personal preference.  You could make it any combination of sizes though.  How many are you looking to serve? 

And yes... you would bake two layers of each size.

Thank you! 

Roughly 20 people. I was thinking about making cupcakes to go along, but would I need them?

You wouldn't need cupcakes.  A 6" over an 8" would serve up to 30 easily but at least 20.  Cupcakes would look cute though!  LOL