My mom and I will soon be going on vacation, she is about 5'7-5'8 and weights 300 lbs, flying US airways. Anyone have any experience flying at this height/weight? I was 272 at my heightest and I am 5'5 and could fly no problem.

I am 5'8", apple-shaped, and weighed 295 at my highest. Planes were TIGHT and I definitely had to stretch the seatbelt as far as it would go. I did not require a seatbelt extender or separate seat, but it was a close call.

at my highest weight I was 285 lbs and I am 5'9". At that weight the seats were a little small so I kind of had to squeeze into them and the seatbelt fit, barely. I'm sure they have seatbelt extenders if needed though.
Have a nice vacation

I think it depends on your shape and the airline. At 5'3" I had a hard time fitting on my last flight and I was around 240ish... it was tight but I managed.

I am 5'5" and at roughly 290 I was able to fly US airways without a seatbelt extender. Seatbelt was tight and seat was small, but I could do it. I am very pear shaped, so most of my weight was exactly where I didn't want it to be (in the seat).

Worse case... US airways offers a last minute upgrade option to First class for anywhere from 50 - 150 dollars per flight. I know from Dallas to Vegas it was 100 to upgrade. You can do this 24 hours before the flight... look on their site for GoFirst option. If she would feel more comfortable it might be worth it to her.

I flew United a few years back when I was around 312lbs/5'6" and I remember it was tight, but I did not need an extender. I'm flying Midwest Airlines in a couple weeks to visit my sister and dreading it a bit... I'm hoping to get down to around 300 before I leave. Anything I can lose the better! I'm considering asking for an extender as I walk on the flight, just in case, although I am less than when I flew the last time and I chose Midwest Airlines because their plane has 18" seats. I may also consider upgrading to first class for the right price. We'll see!