I am planning a trip in Oct 2018 from Nagano to Hakuba by express bus and also from Matsumoto to Hirayu Onsen by express bus.
Can you advise if lugguge bags of 24 -25 inches are allowed on these buses ?
Express buses have luggage trunks underneath the carriage that can store large pieces of luggage. You will have no problem fitting 24-25″ bags in the trunk.
I hope that helps answer your question.