Thanks Terry. (EDIT: You too Peter. I was working on this response when you posted) I was going to go with the 18 on the main wings after you said it as that's what a profile I have scales to, although I don't fully trust it. Now that you said 28, I was actually a bit skeptical and went back and looked closer at the picture of the guys sitting on the wing of Ostrander's plane (see below). Unfortunately, the guy closest to the camera has his leg right over where the edge of the white would be but on closer inspection, you can just see the edge of the band on the underside of the wing (yellow arrow) and I estimate that this would intersect the leading edge just behind the guy's knee. Then I noticed that you can see a partial wing of another Spit in the background (red circle). Although crude, I blew up the picture and calculated the ratio of the width of the white band width to the distance from the cannon to the wingtip, compared that to scale drawings I have and, guess what? - 28 inches!
Now, the band over the fin flash on the tail is another matter as making this a scaled 18 inches obliterates about half of the fin flash. Need to see if I can find more pictures. Any thoughts Terry?