Let me give a brief explanation of my usage to give you guys a better idea.My experience with 15" portables (both PCs and Macs) is that for every time the 15" screen is nice on the move, the other 9 times out of 10 it's a pain in the arse because it's too big to use on aircraft, heavier to carry, gets worse battery life, etc. A 13" machine can be plugged into an external display, (or external GPU, etc.) a 15" machine can't be made lighter, or made to fit more easily on an aircraft tray table.
the 13" retina display is a good compromise imho, you can set it to "more space" setting for effective "looks like" 1680x1050 for a decent amount of desktop space.
Depends how often and where you use your machine away from a desk. Having had machines ranging in size from 10" through 17" (not just macs), i find 12-13" to be the best compromise for a machine that will be used regularly as a portable.
15" is (IMHO) for machines that spend most of their life on a desk or around the house, or are carried occasionally to a desk somewhere else. i.e., they ARE portable, but if you're regularly carrying it around, 13" is better.
Of course the basics like web browsing, emails, notes, reminders, etc.
I import, browse, and manage photos, and also occasionally use Pages for school stuff.
And my most intense use is iMovie around twice a month, sometimes more than that but usually 2-3 times a month or so. I could see that becoming more intense if I get more into video editing in the future, not really sure at this point.
In terms of portability, my MacBook is on my desk most of the time. I will also use it on my bed or around the house, and sometimes outside too. Once a week I carry it out of the house and use it at another desk. May use it in the car occasionally, but mainly desk use. Would the portability really be a factor at this point?