Increase the Maximum Upload Size for File in WSS3.0 by following steps,

  1. Open “SharePoint Central Administration” Site in Administrative tools.
  2. Click “Application Management” link
  3. Under SharePoint Web Application Management, click Web Application General Settings.
  4. In the Web Application General Settings page search the Maximum Upload Size field and change the file size.
  5. Click OK to configure the change.

For additional details regarding this, refer