I am trying to make a RESTful callout containing a very long string (think: the body length of a typical informative email) as a parameter from Salesforce to Marketo. The callout succeeds when the string is shorter, but fails when the length increases. When I make the exact same callout using postman, it works as expected. Is there any limit on the length of the URL (or anything else) I am missing?

The long string contains the HTML body of an email to be sent out through Marketo, a Marketing tool.
Happy to provide details if needed.

I am trying to grab the data stored in a rich text field in its HTML format, and send it via a REST API callout. I noticed that the rich text editor adds tags like <span>, which are irrelevant and unneccesary, and it also adds the 'style' attribute to the <br> tag (ie <br style="">). Lastly, it also adds newlines using \n when I print the html output using system.debug(). [I dont see the '\n', but I can see the string broken into multiple lines in system.debug()].
Has anyone faced this issue, and are there any ways getting around this? Happy to provide details if needed.