How to Fix the '413 Entity Too Large' Error in WordPress
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Note that before modifying any website files, it’s best to do a backup of your website from Site Tools > Security > Backups.
You can eliminate the error by increasing your website limits by adding different rules to your theme or custom plugin. To do that, go to SiteGround Client Area > Websites > Site Tools next to the desired website and click Site > File Manager. Find the themes folder which should be located inside the wp-content folder > themes.

Then right-click on the file functions.php and select Edit. At the end of the file, add these lines:
@ini_set( 'upload_max_size' , '256M');
@ini_set( 'post_max_size', '256M');
Save the changes before closing the file using the Save button on the top.

How to Install Plugins or Themes Larger Than 256MB
In some cases, you may need to install a plugin or theme that exceeds the 256MB limit. This limit is set by the upload_max_filesize and post_max_size values in your server’s PHP settings.
It is best for themes or plugins larger than 256MB to use the FTP (File Transfer Protocol) method. The FTP method allows you to bypass the file size limit by uploading the files directly to your server. This method involves manually uploading the plugin or theme files to your WordPress directory using an FTP client.
You can find detailed instructions how to use FTP to upload and install your themes and plugins in the “How to install a WordPress theme using FTP?” section of our How to Install WordPress Themes tutorial.