The allowed upload size is controlled by the server in the PHP.ini file.
There are a number of php directives that will effect the maximum size file that can be uploaded.
the script timeout limit must be increased to allow enough time for the file to transfer from the client to the server.
The post max size must be increased to allow for the for the file to be posted with the rest of the form.
the upload max file size will also need to be adjusted to allow for the size file you wish to upload.
See the following page for a full list of php settings that effect file uploads:
some of those settings can be changed using the php ini_set() command. for example, to change the max upload size to 8M:
<?php ini_set('max_upload_filesize', 8388608); ?>
the server may be set to restrict these sorts of changes to the php settings though, so you should check with your host for the best way to increase the allowed upload file size.