The archive service and database service actually play nicely together on the same machine because they use different types of computer resources. However certainly after a certain amount of load, you may have to split the load accross multiple computers.
It is difficult to say after X users you need to consider using two machines because it depends so much on how those users are using EPDM and what the hardware is. Though if you are looking for a "belly rub" number I usually say 50 for a typical new server.
File size usually is not a determining factor.