I traveled for about 6 months with a 6.5lb 15.4" laptop. If I wasn't in an aisle seat, just getting it out and on to the tray table in coach was a hassle. Width was problematic (took the whole tray table, or more on RJs). The real problem was depth. To get an appropriate viewing angle the front of the computer was often jammed into my stomach (and I'm not a huge guy). I replaced it with a 4lb 14" Gateway which lasted about a year and now a 14" 5lb Dell Latitude D630. Both are a much better size for plane use (though still not the easiest to whip out on an RJ) and the weight savings is appreciated too (the Gateway was the perfect size but couldn't handle the abuse, the Dell is a little heavier but can take a beating).

All said, 14" is the sweet spot in usable screen size vs. coach seat friendly. If I were in the awful condition of being RJ exclusive I'd probably end up with an Eee.