The best thing is to try them at the store (11 vs 13), but you are likely not going to get a feel for how it is over time.
I found that at home, I'd hook it up to a LCD, elsewhere I would be using the laptop screen obviously.
If you are using it at home, hooked up to a LCD mostly, the laptop screen size doesn't matter as much. If you are taking it with you everywhere and are not home much (i.e. big traveler) then the 11' is good, but the 13' might be better since you're using the laptop screen 'more', if that makes any sense.
Another thing, you can get a 24" LED backed panel fairly cheap these days, $200 or less for something that's 1080p. It's not retina, but it's good if you wanted to do some basic photo or video work, or spreadsheets.
Thanks for the advice; I am planning on picking up a decent 20ish" monitor for around $100 bucks, I have seen some good Asus ones on amazon for that, so it will be perfect for me to hook up and have a bigger screen----but still have that awesome form factor for using in bed or on the go!