Blah. Access has the ability to support many concurrent users, provided you don't use many of the features built into Access (such as bound forms or too much reporting). Depending on how intensive your database usage is (usage volume?), your application can support well over twenty concurrent users. With replication you can have users enter data into several separate databases and merge the data periodically, though this means that until you synchronize, the data may be stale.
In most cases, though, it is probably more cost effective to upgrade to a database server.
As for size, depending on which version of Access you are using, Access supports up to 1GB or 2GB of data. The amount of data is not so much a problem as is how much of the data is being accessed at any given time. So what I'm saying is that your database speed will NOT decrease exponentially with the size of the database, but it will help add to the limit of your file server's ability to grab data from the Access MDB file and send that data over the LAN. At some point your file server/limits set on the MDB file will be swamped, and this is when database corruption begins to frequently occur and the speed slows to a crawl. This is the reason why it is highly recommended that you disable opportunistic locking for Access databases--so that multi-user interferences such as described above do not occur.