Maximum size of post data

That's an understandably frustrating situation. Good news, though. First, the problem is not in the value you're changing: you can make it whatever size you want.  The problem is instead in some OTHER field on that page, which is making the submission fail. And there are a number of possibilities.

Rather than guess what it might be, though, we can find it (even though the message you see doesn't clarify things for you, which can happen for some). Here's a trick: use your browser's zoom feature to zoom out (by default, ctrl - or ctrl-"minus"). You should then see the message. You can then use ctrl-0 to revert to the normal zoom level.

It's a screen resolution problem: I found it happening on a 1080p monitor, where I have the text scaled above 125%. Some laptops even recommend 150%. (Sadly, many folks do find their remote desktop sessions are lower resolution they can't change, even if they may be on a larger monitor.)

BTW, I'd originally proposed this as a workaround: look in the CF logs folder, and especially the application.log. Cause the error then open and go to the bottom of that log. The last line should show the error. But I have since checked and see that indeed such admin errors are NOT logged. The rest of my note below still stands.

As for the problem you may find, it might say that some other field is exceeding its max--such as if you're on CF Standard, and one of the "pool size" settings defaults to a larger number in the free Developer or Trial editions but has a smaller max allowed value, once you put in a CF Standard license key.

Let us know what you find and if correcting that gets you sorted. (I'll also file a bug report about the problem with that "important notice" somethomes not showing the needed details. If/when I do that, I'll report back here with the bug number.)