By default SharePoint allows a maximum file size of 50MB that can be uploaded into any document library. This might be too much as you might not want users downloading 50MB files over your LAN/WAN. To change the maximum file size
1. Goto SharePoint Central Administration > Configure Virtual Server Settings from Virtual Server List Page.
2. Select your server
3. Goto Virtual Server General Settings (in Virtual Server Management)
4. Change the value of the Maximum Upload Size in MB's.
If you increase the file size limit, your site might occassionally time out for users while they are uploading files. To accomodate this increase the default time out property for IIS.