There will be files in your Aces High\Sights folder named [Plane Name].mil (or you would need to create one).
To change the size of the gunsight for a particular plane, you would open (or create) it's particular .mil file. IE, to edit the size of the F4U-1A's sight, you would open F4U1A.MIL
In this text file, you would specify the size in mils you want the sight to be. IE, if I wanted to correct the Mk.VIII gunsight to 150 mil, I would add the number 150 to the first line of that text file. Save it, and load the game.