As one can read throughout most of the articles and guest comments of hotels in Japan, hotel rooms can be very small.  Many of the budget rooms are barely large enough to accomodate a single bed, a closet and a bathroom. 

When travelling with families, these lower cost rooms can be quite attractive at first.  Be sure to understand how many persons can be accomodated in a room.  Often times it is only one adult. 

It is almost unheard of to have 5 guests in a single room.  If you are a couple travelling with more than two children, it will be all but impossible to find a single room that can accomodate your whole family.  Be prepared to book two rooms.  Connecting rooms may not be guaranteed.  Have your travel agent check with the hotel directly.