OK, this was confusing - primarily because the error message was misleading. It implied that it was a configuration setting within in MailWizz. It didn't say that this is a server limitation (and to please contact your admin or superuser or hosting manager, etc.).
I tried to adjust php.ini in the /home/etc folder, but this didn't work.
I happen to be using Virtualmin (Webmin?) on the AWS service and it's in Services -> PHP 7.2 Configuration -> Resource Limits. Change the maximum file upload size to something like 5M or 10M (or whatever you think your max will be).
(Oh, I also noticed that the php.ini I changed was the wrong one anyway - there's lots of them. The one I needed to change in my case was /home/<server-account-folder>/etc/php7.2/php.ini. Your php.ini file might be some other obscure location. And it pays to know how to spell/pronounce "et cetera" correctly: its "etc" not "ecsetera".)