Possibly a bug? I can't reproduce, the panel is keeping the custom size I set it to when I quite and restart Kmail.
It just fixed itself.
But I promise you it was happening before.
Well... Bugs don't fix themselves. Maybe someone read your message, although I would advise using https://bugs.kde.org from now on.
KDE is an international technology team creating user-friendly free and open source software for desktop and portable computing. KDE’s software runs on GNU/Linux, BSD and other operating systems, including Windows.
If you encounter a bug, proceed to https://bugs.kde.org, check whether it has been reported.
If it hasn't, report it yourself.
PLEASE THINK CAREFULLY BEFORE POSTING HERE.
Developers do not look for reports on social media, so they will not see it and all it does is clutter up the feed.