＜Which is better for you? Large or small institute?＞
Giant EIKAIWA chains usually arrange accommodation for you. Generally they pay the key money (a common fee when renting an apartment) and will handle communications with your landlord if anything breaks.
In many cases, they rent several units in one apartment building, so you can feel secure having someone who speaks English near you. This arrangement is really convenient. Having colleagues next door to point you in the right direction or help you get connected is definitely an advantage especially for new teachers.
Smaller EIKAIWA tend to prefer to hire from within Japan, so offering no accommodation is not unusual. This means you have to find your own place to live.
The energy and the cost of finding your own apartment can be several thousand dollars plus a draining experience.
If you don’t want to work for a large EIKAIWA but would like to have accommodation provided, it's worth asking for it at the interview. Smaller EIKAIWA can sometimes be quite flexible.