Solution #1: Add this code to your functions.php
This method will set the values for your php.ini settings to whatever you set below. Remember you must change max_execution_time as well or the script will time out with big files.
@ini_set( 'upload_max_size' , '64M' ); @ini_set( 'post_max_size', '64M'); @ini_set( 'max_execution_time', '300' );
Solution #2: Edit your .htaccess file
You can also edit your .htaccess file to acheive the above.
php_value upload_max_filesize 64M php_value post_max_size 64M php_value max_execution_time 300 php_value max_input_time 300
Solution #3: Create or Edit an existing PHP.INI file
Some hosts make you create a custom php.ini file like godaddy hosting, if you need to do this then create a file called php.ini and put the following in the file. Upload to the root directory of your wordpress installation. Or if you have total control of your php.ini file just change the following values.
upload_max_filesize = 64M post_max_size = 64M max_execution_time = 300