There is no limit to the file size, you can upload through attachmax.
However there is a limit that is controlled by your Server.

If you're on Dedicated or VPS you have full control to configure the server settings for larger uploads but:

If you're on shared hosting, you may be restricted to the settings that are set up by your shared host.

Some of these values can be read about it here

There is a work around that can be done to some Shared Hosts, this can be done by editing the .htaccess files and adding these values.


php_value max_execution_time 600 is equal to about 10 minutes.

One factor that cannot be controlled is Apache Timeout.