Pierre, something in the formatting of your message seems improper. We can see you say, “I did add this”, but we can’t see what the this was.
Anyway, good to hear first that you’re no longer getting the 503. If you mean you got it WHEN you made the change, well yes, if you make any mistake in editing that file, you can find that either Tomcat/Lucee (and ACF) won’t start, or you may get 503’s. Once we see what you added, that may help us see what went amiss.
And as for the “blocking factor for you”, again please clarify whether the change you made was to the http or ajp connector in that file. The default for maxPostSize (if not specified there) is indeed 2mb (specified in bytes, so 2097152), as documented here. To change it to 4mb, for instance, use maxPostSize=4000000 (a 4 and six zeros). Note also that attribute names in that file are case sensitive.