Bus seats are rated for three persons each of whom have 13" wide butts=39" wide seats.
I don't know about anyone else on here but I think each one of my cheeks is 13" wide.
Standard knee spacing is not less than 23" or more than 32".
You will find there are buses that will squeeze 90-passengers (15-rows) into the same space as 84-passengers (14-rows) or as few as 78-passengers (13-rows).
A better guesstimate of length is to count the number of windows. Most windows are about 32" wide.
Some have 9" window openings (which equals 70"-74" headroom). And some have 12" window openings (which equals 74"-78" headroom).
To guess which size window openings a bus has look at the height of the windows in relation to the top of the service door and driver's window. If the top of the passenger's windows are equal to the top of the service door and driver's window it most likely has 9" window openings. If the top of the passenger's windows are in a line above the top of the service door or the driver's window it most likely has 12" window openings.