What I have found is that a general subjective estimate (read guestimate) is 2.5' per window including pillars from the inside back door to the threshold and from there it's 9' for a CE to the front bumper.
They are generally 7.5' side to side on the inside.
If you search on Google, you can get some pictures of more detailed measurements but they only apply to the drawing. Your bus will be different.
https://www.google.com/search?q=scho...w=1680&bih=953