It’s recommended to pack lightly when travelling in Japan as the majority of trains do not have enough dedicated space for large suitcases / bags. There is very little room for larger luggage on long-distance trains such as Limited Express and Shinkansen (Bullet Trains). However, there are some exceptions such as airport lines which have designated space for suitcases.
According to JR regulations, each passenger may bring up to two pieces of luggage onto trains, not including smaller bags. Each piece of luggage may not weight more than 30kg and its three dimensions (length, width and depth) may not add up to more than 250cm, while its length may not exceed 200cm.