This is a brief study to set the record straight, as I see it. Somebody at Lucasfilms made a mistake, back in the day, and we are now paying the price. The mistake? An incorrect quoted length for the A-wing of 9.6 meters. The price? Star Wars Rebels actually scaled their version of the A-wing to those dimensions (as did FFG) and it looks silly! Just look at the cartoon pilot swimming in his luxurious cockpit. You could fit three people in there, plus a Wookiee! That's just one of the latest attacks on Star Wars perpetrated by the Rebels people. More on that, later.
Many have tackled this subject before, some incorporating screen captures and relative size comparisons, etc. Here is one such discussion from FFG discussion boards, and one from BoardGameGeek (and another, for fun).
My argument relies on simply two variables. Observed cockpit width (inferred from some movie stills) and the ratio of that width to the overall length of the craft.
First, cockpit width. Here are some legitimate, actual A-wing pilots for you, taken from the original source: Return of the Jedi. Behind the scenes, they are probably being filmed in the same A-wing "set", I'd imagine. Arvel Crynyd, Jake Farrell, and Sila Kott.
I submit to you that these three are fairly cramped in the small A-wing cockpit. Measuring myself across the shoulders, I took a guess that this is something like 2 feet across. Modern jetfighters (more on that in a minute) agree very closely with this "shoulder width" measurement guess.
Here is a drawing of the A-wing where we can get the ratio between the shoulder width and the overall craft length. I drew a orange highlighter line across the area of the cockpit where I suspect the pilot's shoulders are located (halfway back in the "rear section" of the canopy). On this particular printout, that shoulder width measurement was 18mm. The overall craft length was found to be 185mm, for a craft length of ~10 times the shoulder room measurement. Ten times two feet is twenty feet, or ~6.1 meters.
Working backwards, if we actually think 9.6m is the correct ship length, we need to assume that the shoulder width allowed for the pilot is 3 feet. That is a very roomy cockpit, much like what Star Wars: Rebels illustrates incorrectly.
Just for a proof of this "shoulder width to length" concept, I applied the same technique to a real world aircraft: the F 16 Falcon.
The Falcon pilot is very cramped, too. The cockpit is barely wider than their shoulders, at something like the 2 feet I guessed for the A-wing.
Applying the same logic to another scale drawing suggests that, in the case of the F 16, the shoulder width to overall length multiplier is something like 23 times.
This suggests the overall length of an F 16, just based on the apparent shoulder room for the cramped pilot, is something like 46 feet (14.0m). The actual length of the plane is 49.4 feet (15.06m). Very close, especially for such a rough method of guessing, based simply on how cramped the pilot is while flying.
Long story short: the A-wing seen in Return of the Jedi is not 9.6m long. Rebels and FFG have incorrectly repeated this mistake, and made the vessel much different from its origins. In the case of the FFG miniature, it is a faithful 1/165 scale reproduction of the A-wing (not 1/270 they were shooting for). The A-wing made by WotC for their Star Wars miniatures game is much closer to being correctly scaled, and with the advent of the FFG spare bases and pegs accessories, I have made the switch to the more correctly sized craft. I will keep the beautiful, wrong scale FFG ships somewhere safe, off to the side.
I can't resist a few more pictures to drive the point home. Note the size of the pilot in relation to the overall craft. This is not the annoying Rebels version of the A-wing. This is the original item (with a British Royal Family shot tucked in for good measure).
To be complete and as fair as possible to the Star Wars Rebels depiction, here is the prototype (?) design by the great Ralph McQuarrie. I suppose it is among the first views of the fictitious craft, and it looks & feels larger than the ship that eventually appears in Return of the Jedi. I admire how Rebels often goes back to the old McQuarrie stuff for inspiration (it is the best part of the cartoon, in my mind), but don't do such a thing if it opens up a discrepancy with the OT films.