I'm coming into this conversation late, so I'm likely missing something, but...
If you want to go with size, get an MBA. It has a much better price vs. performance ratio.
To me, the 13" rMBP is a complete waste of money. It's not as thin and sleek as the MBA, and not nearly as capable as the 15" rMBP. All in all, the 13" is a pretty crappy low end of the mid level laptop with a really nice screen.
...and that really nice screen won't amount to much because it doesn't have a graphics card capable of taking advantage of it.