Luggage services/Taking the bus

I want to stay 1 week in Tokyo, then move to Sendai OR Osaka for one week and then go back to Tokyo for another week. I'm aware there are luggage shipment services but I'm wondering if it is easier/cheaper to take a highway bus for that round trip. I'm going to have a rather large rolling duffel bag and I know the overhead bins on the shinkansen won't be big enough for it. I'm going solo and it's to/from hotels.

Shinkansen and long distance buses can accommodate large luggage.

On Shinkansen, at the rear of each car is a space where you can stash large luggage. Actually, the overhead shelf on Shinkansens are quite spacious, too.

On long-distance buses, large luggage are stored under the carriage - it's huge like a cave down there.

My bag measures 77x40x40. Would that be small enough to fit? It is a rolling duffel bag.