For sizing a rough ball park is 500K per average active session per hour.
A small but active database would be 1 active session
A medium would be 10
The biggest databases in the world are around 100-300 active sessions
In general there are about 1/50 of connected sessions as active sessions, though of course your mileage may vary quite a bit.
For example if I have 200 users connected I might see 4 active sessions on average, and thus do about 2M of data collection an hour.
Another fact though is we collect SQL text. If the application has a lot of SQL text that will increase the amount of data. If the application has a small set of SQL then we collect less.