How big can a carry on be for Greyhound?

They don't give how big a carry on is suppose to be, they just say it must fit. My husband has a mini suitcase thing for his carry on but I'm scared it will be too big and have to go under the bus and charge us more money, which we don't have. So if ANYONE can give us example of how big the overhead compartment is and below the seats room there is so we can know.