In the root folder of Joomla! there is a file called htaccess.txt rename this to .htaccess. If you have already renamed the file then continue....
The PHP value you need to add is
php_value upload_max_filesizeSo to allow a max upload of 8 Megabytes you would add to the .htaccess
php_value upload_max_filesize 8Mbest practive suggests you add this near the top of the .htaccess file. You should not set this to a HUGE value a good value would be no more than 10M (Megabytes).
Note that this will not work on all servers, verify with your hosting provider if your server uses a local php.ini, .htaccess or disables the setting of PHP values.