The bug can be reproduce:
1-kill plasma
2-Remove all screens leaving only the smaller one
3-Launch plasma
4-Add a new bigger screen

The panel will have the old size but will be placed in the right place.

The bug seems to be in the if/elseif block's that check conditions suck "The old screen was smaller, checking...". The first conditional block is to know if the config has to be saved, if the config has to be saved then none of the checks will be perform since they're in else if, so the panel won't be resized.

I have been messing around for 1h including vertical panels, so far so good.