[Alex Wilcox]

It totally depends on the model of the bus unfortunately - Out team actually has to make sure that the bus we get is one of the taller ones because we have made stuff to fit the compartments. The only consistent measurement is the width, highway coaches are 102 inches wide so you can usually get an 8 foot item across inside, but there are usually gas struts and latches in this area that may prevent larger items from packing at 8 foot width. 7' 6" should guarantee a fit side to side.
Front to back, the compartments are usually 4' or greater, but this depends on the bus. Height is the biggest variable, Prevost X3-45 coaches are the tallest and others are shorter from there.
Call them and ask for dimensions, and then be aware that there may be other obstacles, such as power inverters and wheelchair lifts, or bus related stuff (tools, lav fluid, cleaning supplies) mounted or stored underneath as well.