Just as important, what are you trying to accomplish?
School bus windows are pretty well standardized, based on the also standardized spacing between the seats. More seats; more windows of the same kind. My Blue Bird had (approximately) 28 inch spacing. Then subtract the pillars and framing and you get somewhere around two feet of glass. This probably varies a bit between brands, but you asked for ballpark.
Just guessing here... If you are trying to estimate the length of a bus by counting windows, you won't be far off by multiplying windows by 28 inches.
That said, some buses have one window that is different -- usually longer. Looks to me, this happens in order to compensate for some mismatch between the body and the standard window count, such as when an emergency door upsets the regular seat spacing.
Give us more to work with, and we'll help you all we can.
Oh... by the way...
Welcome aboard the skoolie forum!