It really depends on usage. My mom has a 17" MBP, and I have a 13" MBP.
For browsing the internet, watching movies in the car, and general typing, I find the 17" to be cumbersome.
For working with Excel, Photoshop, Illustrator, Sketchup, and any other productivity software, I find the 13" to be too small.
I think that the 15" with the HR AG is the nearly perfect compromise. After using all 3 side by side, the HR AG 15" is nearly perfect for my usage as an engineering student that likes to dabble in graphic work in my free time.
I would wait though for the rumored refresh. A 15" MBA is rumored and that could be the computer you're looking for.