This document will provide you with some information on why you are receiving a 'Request Entity Too Large' error, what it looks like, and how to fix it.
You will receive the Request Entity Too Large message if your page content exceeds the limit (260,000 bytes). This is an indicator that your page is too large and cannot publish due to its size. It's a common issue when adding too many accordion panels to a page or when a form has too many fields/conditional logic, but it can happen when there is simply too many fields and text blocks on the page, as well. Unfortunately, you, as a user, will not be able to see when you're nearing the limit so it is always recommended that you shorten very long pages and forms as much as possible. This error is an indicator that the limit has been reached.
In order to fix the issue, you will need to cut down on content. For example, if you have multiple accordion panes in one page, consider splitting out the groupings into separate pages. For large forms, consider removing unnecessary content.