I had similar worries when, at 5'5" and 20 stone +, we flew to Lapland. It was an Xmas treat for the kids, and my excitement beforehand was really marred by worrying whether the "provided" snow suits would be big enough (they were) and whether I too, would fit in the airline seat.
I even had DH Google the planes that the company flew with, and he made a "seat" out of the dimensions he found, which I practised sitting on, to ensure my vast btm would fit. It did, just, but was snug and uncomfortable on the real thing. I managed the seat belt because I tucked it under my belly. (Oh how horrid...)
Since then I have lost about 6 stone, well more but then regained some! We have just booked for Florida for Easter 2009, and this time there WILL be room to spare!!!
So you see, you are not alone in your worries. And with the support of all these fabulous Minis, we wont have to worry again!!