Subject: Long Trip to Japan

I will be living in Japan from February until August and I need some experienced advice.

For the first two months I will be traveling extensively from town to town and need to know the size of luggage I would be able to travel on the trains, busses and subways with. I will be storing most of my luggage at the school I will be attending until April. I know that for the most part luggage is shipped separately to your destination because of the public transportation congestion. Would I be able to travel with a large backpack? Well I also have my itinerary planned out but I need to know if I must make all my reservations in advance or if I change my plans and be more flexible?

Besides this any other advice?

Thank you for your time, David