Huge and richly decorated in Christian symbolism.
Global Orthodox was at the World Russian People's Council meeting last month at the Russian Orthodox Church's enormous conference / exposition center which is part of the Christ the Savior Cathedral complex two blocks from the Kremlin, and we took some amateur video on our phones because we thought it was a very cool space.
The cavernous center is built into a hill sloping towards the river from Christ the Savior Cathedral, Russia's largest and the home cathedral of the Patriarch, and reaches into the ground under the cathedral. It is enormous, has a large museum, many conference rooms, and full catering facilities.
Here is the main hall, with the head of the Russian church, Patriarch Kirill and conference deputies singing the national anthem:
Some more of the interior:
One of the seemingly endless quantity of conference rooms in the back":
Artwork on the walls: