It is not uncommon when logging into WordPress admin to see visual anomalies ini the main content box for posts, pages and custom post types. A typical issue will be seeing enlarged text or content only in half of the textarea, an example of which is text as shown below.

half_error

The simplest way to resolve this is to remove the following function, often present in many themes functions.php file

add_editor_style();

This function is a callback for bespoke TinyMCE stylesheets. View the WordPress documentation here.