The limit for the file size is the size of your hard disk unless you choose to implement some sort of Quota.
I'll assume you mean http sessions, although Alfresco also has other sorts of sessions such as cifs or ftp. The maximum number of sessions is a large number unless you choose to control it through your application server (e.g. Tomcat's maxActiveSessions.)
In sizing your system you will probably be more concerned with what the users are doing concurrently. In general the limiting factor is the amount of RAM available. There must be a limit where your hardware and memory become inadequate and you will need to cluster your system.
