The official limit is about 5MB (although not generally strictly enforced). We are about to roll out new MySQL plans which are based on both size and the amount of connections you make / the number of selects you make.
This means that we can significantly improve performance for customers with lower use patterns, and charge customers with higher usage more. This also encourages people to optimise their code, and keeps our main shared MySQL machine from being overloaded.
Not to fear, though. The customers likely to be affected most by this are a very small percentage of overall MySQL users.
As far as space; if you’re not using an obscene amount of space I wouldn’t sweat it too much.