To increase the file upload size of a site following variables need to be configured

memory_limit 64M [PHP_INI_ALL]
max_execution_time 600 [PHP_INI_ALL]
post_max_size 16M [PHP_INI_SYSTEM|PHP_INI_PERDIR]
upload_max_filesize 12M [PHP_INI_SYSTEM|PHP_INI_PERDIR]

You can change them in php.ini or htaccess

Now regarding these variables you will find in the php mannual which can be set where.
Bellow I have included the categories

PHP_INI_USER [Entry can be set in user scripts] 
PHP_INI_PERDIR [Entry can be set in php.ini, .htaccess or httpd.conf]  
PHP_INI_SYSTEM [Entry can be set in php.ini or httpd.conf]  
PHP_INI_ALL [Entry can be set anywhere] 

So that means you can set the values of “memory_limit” and “max_execution_time” in
your script using ini_set or in htaccess using php_value or in php.ini and the other two
“post_max_size” and “upload_max_filesize” have to be configured in htaccess using php_value or in your php.ini.

Please make sure that post_max_size is greater then the upload_max_filesize
because in your form post there will be more items then just your file.