Historically for the British we have 3 flights of 5 + a normally non-flying Squadron Commander.
The Germans 2 Kette of 5 + a flying Jasta Commander.
For the French: ?
Americans: Same as the French?

And I will add this to the Wish List but I wish the British could list 3 flights (and the French and Americans if applicable) and that there was some kind of persistent "flight integrity" for all nationalities instead of the one or two random groupings of pilots for missions drawn from the squadron personnel pool as in the current campaign structure.

http://albindenis.free.fr/Site_escadrille/escadrille_type_1914-1915.htm

for the French. Maybe Corsair can put a few words to it

AFAIK, for the french, there wasn't this notion of "flight", looking in escadrille diary you found that some "leaders" flew with different wingmen each day, even if you can sometimes notice that some pilots flew a lot together. In French escadrille, the CO also fought in the air.
Size of escadrilles changed during the war, and vary depending on their mission and affectation.
Basically an escadrille de chasse in 1918 had between 15 and 20 planes.

Sounds like WOFF, in its current form then, models the French system nicely.