Tokyo Dome

Tokyo Dome and Tokyo Dome City Hall (Formerly JCB Hall)

Tokyo Dome (wikipedia)

Seats: about 55,000 (depends on the layout of the arena seats)
Stands seating chart (Japanese)  /  English chart  /  English map of the Gates
Arena – varies, see more info here

Access:
JR Suidobashi Station (Chuo-Sobu Line). A few minutes from either exit. The east exit (東口) is closer to where the goods line usually starts/ends/passes (it sometimes goes through Prism Hall). But the west exit takes you by the restaurants and a bookstore that will put out magazines, etc., featuring the idols, sometimes even older back issues.
Mita Subway Line - Suidobashi Station. Take any exit between A2 and A5 (see map).
Marunouchi and Nanboku Subway Lines - Korakuen Station. Take exit 1 or 2.
Oedo Subway Line - Kasuga Station. Take exit A1.

The Tokyo Dome City Hall (Formerly JCB Hall) is located inside the Meets Port Building. You can use the directions and map above to find it.

Seats - Around 3000, depending on the arena layout
Stands Map