The database size just counts toward your account's disk space quota I believe. The only other limit we have on it is overuse/abuse of CPU time. Running complex or badly written queries that monopolize the server (or crash it...) can get you suspended for high load. Pretty much all common software doesn't have any issues with the load though. Finally, I recommend staying away from the innodb storage engine in MySQL if you're on Stevie, it is subject to going read only or just losing your data without notice if the database server crashes (can happen due to abuse on occasion).