I would check with your Travel Agent now - if you leave it until you get to the airport the upgraded seats may be all gone. You will need to arrange extra legroom for the return flight too.
My wife and I went on this cruise last November. Abolutely first class, you'll love it.
As for the leg room, I am over six foot tall and, like you felt that I needed extra leg room. I upgraded only to find that extra leg room seats are those placed in front of a bulk-head so although there is technecally more room between the edge of your seat and the bulk-head, you can not put your feet anywhere, so you are in fact worse off. With standard seating, at least you can tuck your feet under the seat in front of you but the extra leg room seats make you sit with your knees permenantly at right angles to the bulk=head. Most uncomfortable.
If you need leg room seats for medical reasons, i would suggest that you get a letter from your doctors surgery stating the reason. then ring up princess cruises and they will ask you to fax the letter over. they will then submit it to the airline who will then look at your letter and other letters which have been faxed over. they will then decide who needs leg room seats more than others, unfortunatly they cant tell you whether or not you have been allocated one until you get to the airport. but usually medical seats are free.
Can understand your concerns but I am 6 feet and heavy built - I manage without too much discomfort - try checking in on line the minute the flight is available and grabbing the two back seats [rather than the three across] in economy or two of the seats at the front of economy where the legroom is more flexible. Be careful if you have a disability that you don't get the emergency exit because you can't sit there if you are unable to handle the emergency exit doors [I would think in an emergency nothing is going to stop me using the emergency exits!!] Don't worry about it - you're going on holiday.
If you are going from Manchester there's a good chance you'll fly with Thomsonfly. The seats are spacious and are in a 2 by 4 by 2 formation. The 2 seats together are a better option. Make sure you check in early - no later than 3 hours before!
However if you're flying with Thomas Cook - good luck! There have been an abundance of complaints about lack of legroom on their flights, which going by the complaints have all been from Gatwick.