I see no one answered you.
I had a look at the Official Monogram Guide, and it is a long story. Standard letter sizes for most applications were 8, 12, 16, 20, 24, 30, or 36 inches high during that period. However, sizes of 9 and 18 inches were specified for some uses. Also, while some sizes were standard for particular legends or titles regardless of aircraft type, others (such as the base codes on starboard upper and port under wing surfaces) were specified to be the largest standard size that the airframe could accommodate. Basically it depends on the plane, the role, the time, and which lettering you mean.
Anyway, here's a link to the book you want.
http://www.amazon.com/Official-Monogram ... 0914144332August