It is difficult to come up with a particular number. It really depends on
how well the application is written, and how well of a user environment
everything is run under.
If after compacting a mdb file, (no forms, reports, but just tables), you
have a file in the 500 meg range or more, that is getting rather large.
However, 500 megs is not too big for a single user when no network is
involved.
The same goes for the number of users, it will much depend on how reliable
the pc's on the network are, and how well the application is written.
however, when you get beyond 20 users, then likely the value of the data,a
nd the value of a work stoppage failure gets quite high. I mean, one person
working and entering data represents one person day of data. However, if
you have 20 users, and a file is damaged, you now loose 20 persons days of
productivity. You have weigh this value.
On a good network, with stable pc's, you can easily run 20 users, but the
value of that many users means that sql server can also be justified. I
can't imagine a company reaching 20 users with 20 computers. At 25
computers, you generally be close to having a one person FULL TIME support
that many pc's. Hence, the risk of data loss means that sql server can be
justifed with ease.