For letters sent Airmail, size isn't really an issue (unlike internal mail). However, length, width and depth combined must not be greater than 900mm. The greatest single dimension must not exceed 600mm. I doubt it will, but there you go ;) Don't forget to write "By Airmail/Par Avion" on the top left hand corner. There's no need to put a return address, since the address on the front will be your own anyway (they're only used if it's undeliverable i.e. they cant find you).
Under 10g: 56p
Under 20g: 81p
Under 40g: £1.22
Under 60g: £1.64
1st class (gold) stamps are worth 36p, 2nd class (blue) are worth 27p (the "classes" are only important when used internally, so dont worry about them). Make sure you don't get the regular stamps mixed up with the large ones (they say "large" on them, again this is only an internal mail issue... but they are worth more). You can get stamps of varying pence value to make up the difference if you want to (rather than use 1st or 2nd and pay over the cost).
Favourite: David Tennant IP, John Barrowman IP, Patrick Stewart IP, Catherine Tate VV. Newest: John Simm IP, Suranne Jones IP, Jonathan Groff IP.