If you're close to ultralight (<10 lbs base weight) pretty much anything should work. I've carried on a large Golite Pinnacle with a week's worth of food (my base was almost 12 lbs). It could fit in some overheads (ie, 737) that way but not others (little regional planes) so I stuffed it under the seat. Had my one change of clothes in my laptop bag that I left in a locker at my destination.

Forgot to mention no need for a rain cover. Put your critical items (sleeping bag and sleep/puffy clothes) in trash compactor bag and other items you need to reach easily (eg, electronics) in ziploc.