Hi Ian. The concrete squares varied in size, depending on circumstances during building, I guess. On average, they were between 8 x8 feet and 12 x 12 feet, but don' take this as gospel, as some could be larger, or rectangular. Wimpey have a web-site about WW2 airfield construction, with lots of facts and figures; it might include this info, but I doubt it.
As for aircraft on grass, again, that would depend on the airfield the type was based at during the period depicted. For instance, most bombers in the Wellington/ Whitley class up, were based on pre-war expansion period airfields which, although many still had grass runways, had hard peri-tracks and dispersal pans, and permanent hangars and buildings, eg Scampton, Dishforth etc. Note, however, there were a few exceptions. Airfields built during the war (bomber fields), all had hard-standings, due mainly to the increased weight of such types as the Lanc, Halifax and Stirling.
The easiest way to ascertain what type of dispersal is needed for your dioramas is to let me know the following;
Squadron, approximate date (month/year) and aircraft type.
From that, I can provide you with the airfield name, a description of the hardstanding (if any), and probably a diagram or sketch of the latter, and possibly an airfield plan.
It's no problem, as I have the referencexs to hand whilst working on my Diorama Guide. PM me with an e-mail address if it's easier for you.
Terry.