That is a server issue. In addition to the vB settings, the allowed size of the attachments depends on the PHP and MySQL configuration. You may need to check and change the upload size for both PHP and for MySQL. Note, you'll need to have root access to the server to do this (or have your host do it.) Make these changes to php.ini:
upload_max_filesize = xM
..and my.cnf (or my.ini for Windows systems)
set-variable=max_allowed_packet=xM
Change it to the size ('x') you want in Megabytes. Restart the webserver and MySQL after making these changes.
Note: Do NOT use MB, just M for the megabytes setting.
You may also need to increase the memory_limit, max_execution_time and max_input_time variables in php.ini.