is there a max size to the length of a hidden input in html?

Question

in other words:

and if so, what is it?<

Answer 1

It depends on the method you send the form with.

With GET, there is a commonly agreed on limit of about 1-2 kilobytes, depending on browser and server limitations.

With POST, there is no technical limit in the browser, but usually one on the server side - see e.g. Apache's LimitRequestBody, PHP's post_max_size and so on.