The size you can run an access database depends on a number of things.
As Access was not designed to be a web backend database system and as it is a flat file database the performance of it when used for web applications is not great.
If you are on your own dedicated server with no other sites on the server and a powerful server than you can have quite large access databases.
If you are on a shared server than depending on the power, memery available and the number of sites on the server 10mb can be considered very large, as each time someone connects to the forum the whole 10mb file needs to be loaded into memory. If the server is busy, or if someone on the server is using bad programming and has eaten up allot of the server memory then the forum with an access database will run slow and give timeout errors as it takes quite allot of server resources to open an access database using the JET database drivers form an asp application.
If you find that your forum runs slow with access then you have two choices, get a dedicated server, or switch to SQL server. I would go for the SQL srever as it is much cheaper.