Hi Anne,
It is important that your sleeve fits well as an ill-fitting sleeve can actually cause Lymphoedema. Seeing a Lymphoedema therapist is a good idea as they can also give you some hints on other things that are important in preventing any problems.
In my experience I have been advised to drink plenty of water before, during and after the flight. Get up and move around as much as possible and also do some simple arm exercises so that you don't have your extremities swell too much.
If you do wear a sleeve it is also recommended that you leave it on for a couple of hours after your flight.
Hope you have a great time on your holiday. Take care. Deanne xxx