jaquadis,
12" & 15" were the issued specs, but if you want to faithfully reproduce a certain aircraft I'd try to find some photos to refer to. The shot below illustrates why:
For a larger image, click on this link:
http://photopile.com/photos/blackwolfscd/FSM/14426.jpg
As you can see, there's variations of the ID bands on various aircraft. Often times the stripes were ragged and of an inconsistent width, especially if applied in the field.
Now,
most of the ID bands I've seen are uniform and straight, but this shot illustrates that there's an exception to every rule!
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