Q2A does not impose any limits of number of users, connections or size of database. You will have to check with your host in case you have any disk space limits, but you should be fine.
As a guide, my installation which has been online for about 2 years now, with 10,410 questions, 23,436 answers, 29,861 comments and 5,831 users takes up around 250 MB of space (the majority of which is the indexing tables for searches). And it's still as fast as it was day one.
