
As a general guide, you can quite safely assume that only cabin luggage size bags will fit in medium size lockers, anything significantly bigger you will need a large locker.
Note that while busier stations like Tokyo have more lockers, they are also tons more people. My last trip it took me 25mins dragging my bag all over the station to find an empty large locker.
Note also that Tokyo gransta mall has a cloak room but I no use it before.
large locker hard to find
medium size easy to find