Really? I don't mean how big of a bag do the airlines say can fir in one, but how big of a bag COULD fit in the overhead bin? I have a bag that is about 1inch too big, and was wondering, will it be okay? For say, a 767 or A320 I wouldn't worry about it. You should see the size of the bags some people drag onto the airplane! The airlines want it small enough to fit in lengthwise, but if it doesn't fit that way, just turn it sideways. The bins on the A320 are pretty spacious, the ones on the 767 are a little more difficult because of the way they are shaped....but your bag will fit...it just might be the only bag to fit into that particular bin then.