The rMB and 11" MBA are extremely close in footprint. The rMB is a tiny bit deeper and a bit narrower but a soft case for either will fit the other so in general where the 11" MBA fit, the rMB will fit and vise versa.
I fly a lot and have never had trouble with my MBA or my rMB on a tray table of a normal size jet. Some commuter or regional jets are marginal (fit is fine but you might not be able to tilt your screen back enough to get a good viewing angle).