The whole Congress Center can host event with more than 9000 participants. It's largest hall, designed as huge theater with two balconies, has seating capacity of 2800 and features exceptional acoustic. There are four other large halls with flexible configuration and capacity of thousands and 15 smaller halls. You do not have to book the whole complex, renting any part separately is always possible. World class accommodation and catering services are available.
The Congress Center offers magnificent views of the historical Prague city center. It is built at the edge of a plateau, facing a deep valley over which the Prague Castle skyline can be seen. If tired of conference, one can conveniently walk around the surrounding park and medieval complex towering above the river. Or take a subway and arrive to the city center in five minutes.
Yet there are several drawbacks. The Congress Center was built in the 80's of the last century and only partially modernized during the last decade. There is still look and feel of the old times here and there. Service is sometimes not as flexible as you would have expected. However if you need to host an event with more than 2000 participants in Prague, you have hardly any other choice. And the price is not bad, compared to major hotels.
More to that, the City of Prague has decided to support major international events that bring foreign visitors to Prague. The City offers a subsidy of 10€ per participant to organizators of conferences with more than 1500 participants held in the Prague Congress Center.