It's largely a taste thing I suppose and what looks right to you. Where aircraft are on the ground, it might look a tad odd if the aircraft model is way bigger than the vehicles and troops, but if they are a flying 'token' (after all, how long would it take a modern jet, or even helicopter, to pass over a battlefield, even in 6mm?), then it isn't so important.
A 1/72nd scale F-100 is just over 8" long, while a 1/48th scale one would be 12.5" long (1/100th would be 6"). Presuming we are talking 28mm figures, then it should be around the 10" mark. A CH-34 Choctaw is 14" (1/48), 9.5" (1/72) and a touch under 7" (1/100), while for 28mm figures it should be around 12". A Blackhawk is 16" at 1/48th and the model on the magazine looks easily the almost 11" it is supposed to be at 1/72. The measurements are largely meaningless sat here, but when you see the size of the models themselves, space becomes an issue and 1/48th models look very big when we're talking modern aircraft.