Well, there isn't only one answer for this questions. For me putting all the tables in 1 db was better. Now it has about 10 MB and growing, and I did not notice any problems in speed or so. I have there maximum 200 vistors/day. I have had an Access DB of 46 MB, but the site wasn't much visited (less then 50) so I don't know if could handle a serious trafic.
Is not much the db that realy counts, but the code. If the code is lowsy chances that the db would crush grow accordingly. If you plan to have a heavy trafic site, you'd better consider MS SQL or MySQL. They are much better equiped for web usage.