IM@S is one of the handful of anime events that is getting a dome live. Seibu Dome is actually out way in Saitama and takes about an hour from Ikebukuro station to get to. Here are some details and links.
http://www.seibulions.jp/ – Official site of the Seibu Lions, who play there. They have a home game on the 17th of July, but playing at Tokyo Dome.
http://en.wikipedia.org/wiki/Seibu_Dome – Note the name changes. It’ll be the Seibu Prince Dome come March so… i think official capacity of Seibu dome is like 39K, but for concerts that number will go up to 40-50k depending on how much space they have to block and open up on the field.
http://www.stadiumjourney.com/stadiums/seibu-dome-s968/ – a baseball traveler’s guide. It’s pretty spot on in terms of seats but remember for IM@S the field (arena) will also have seats, which changes things a lot. The semi-exposed dome is pretty good ventilation if you are up in the main walkway. Seibu Dome is unusual in that the entrance is from the top and all the seats are below, and it’s on a slant where the outfield is at the lowest position.
Seibu Dome has its own train stop, and you have to make a transfer from Seibu Ikebukuro line to the Seibu Sayama line at Nishi-Tokorozawa. Since this isn’t JR your JR pass won’t work, prepare something like 1500 yen for round trip fare. Usually the transfer is on the same platform, so it ought to be easy to do. Some details here with map and English instruction.
There is a Family Mart right across the street from the station, which will run out of water before a live. Japan in July is humid and hot. You will be out of direct sunlight for the most part of the live but it is an open dome, so the temperature will not be cool. At any rate, please remember to hydrate yourself at the event. You can bring fluids into the event (regulation conforming) so do take advantage of it.