Locker 13 x 34 x 57cm 600/day near Tohoku Shinkansen entrance

Full service cloakroom 500 for the day, must be retrieved by 8pm, total dimensions L x H x D 200cm

http://gransta.jp/cloak/index.html

The last reply may have left a digit off the size of the locker. Large lockers are approximately 117 cm x 43 cm x 57 cm: japanrail.com/JR_atthestation_coinlocker.html

If you don't need a large one, there are lockers in many locations in Tokyo station:

http://www.jreast.co.jp/e/stations/e1039.html (this links to a hard-to-read html, but it has a link to a pdf)

There is a left luggage facility in Tokyo Station.

http://tinyurl.com/6lpj9s

Look at reply #2.

Obviously Roger's dimensions are correct. Mine was quoted from a post in Yahoo japan.

Many times, the lockers at Tokyo Station are all full, especially on weekends during the day. If you go downstairs towards the NEX/Yokosuka Line platforms, one level above the platforms where the toilets and convenience store are, you can find plenty of lockers in every size along the walls. Those lockers never fill up and there are always plenty available down there, even during the busy season.