What are the size, in fighters, of the following designations:

flight
squadron
wing

Any help and where you got the info (sources) is appreciated.

Regards,
Matt `da wolf

Squadron is 12 Fighters
Flight is (I can't remember)
Wing is 6 Squadrons; 72 Fighters.

I got those from memory from reading the Essiential Guides to Ships.

Flight I think is the lowest unit, consisting of about 4 fighters. (pretty sure of this, but don't quote me on it. :D )

Squadron is the next unit, and comprises of 3 flights, 12 fighters.

A Wing consist of 6 squadrons, totaling to 72 fighters.
A standard Imperial Star Destroyer carries a full wing.

Read the X-Wing books, they cover this. ALthough theses people have already given you the answer.

A Flight can be split up into either 3 or 4 fighters

As has been said: read the X-Wing series.
However, this designation for squadron is only standard and official, not necessarily how it always works. For much of the New Jedi Order, squads were smaller than this simply because there weren't enough pilots. Likewise, wings varied in size, also b/c some groups referred to as wings didn't follow the NR/GFFA standard designations...

In the early Rogue Squadron comics, they usually had around eight pilots due to the difficulty of finding experienced pilots, plus all the deaths.