Will I fit in a first class airplane seat?

Airlines have policies for passengers who are larger than their seats, whom they most commonly refer to as "customers of size".

Economy airline seats generally resemble a thin couch for three people with hard armrests between each seat. The armrests between the seats raise up and you can sit across two seats though it might not be that comfortable depending on how high the armrest goes and the ergonomics of the seats. First class seats are often more separated from each other.

Some airlines ask you buy two adjacent seats, and you can get a refund for the extra seat if the flight isn't sold out, while other airlines don't require you to buy an extra seat but will ask you to wait for the next flight an empty extra seat if the one you booked is sold out. You can find more info on some standard policies in this airfarewatchdog blog post.

You'll be best off contacting the airline directly because the seat size depends on the aircraft they use; while you may be ok in first class on an overseas flight, a little regional jet's first class seat might be too small.

From the US Airways web site:

Aircraft seating configuration

For information about aircraft seating configuration, please the map shown during the booking process on our website (usairways.com.) Customers may also request information regarding our general aircraft configuration at any time by calling US Airways Reservations at 800-428-4322. US Airways agents are able to provide information regarding exit row seating, seating for passengers with disabilities, seat configuration, average seat size and pitch. Telephone reservations agents, upon request, will provide the customer with seat information including the average seat size by class of service. The information is accessible in the Direct Reference System through the computer reservation system.

So in summary, it depends on the flight, and the aircraft. When you are booking your flight, you can see the size of seats while selecting your seat. If you want to know ahead of time, it's best to contact the airline directly.

I have two very tall sons - 6'4", 270 lbs and 6'6", 240 lbs. The taller one has legroom issues, but fits snugly in the seats. The 6'4" 270 lb guy has more trouble. He can fit "in" the seat with his hips, but his very wide shoulders hang over into the seats on either side of him. He tries to sit by the window, as you can kind of sit a little diagonal, wrapping a shoulder up in front of the window. Honestly? I say take the wonderful lady on a cruise - you'll both be more comfortable!