I host a Synapse node with >400 users. The database (without media) is currently >500 GBytes. The handling of state in Synapse is just broken (the state table alone is over 370 GBytes big).
I hope for you that Dendrite behaves better in regard of DB size. :)
matrix.data.haus is sunsetting in 14 days now. I've set up a small Prosody XMPP server for me and my friends instead. Seems to work a lot faster.