*Please* remove all panel height dependent *fontsize formatting* hacks from label! It's make a chaotic mess, it's unreadable when panel fontsize is small. If I want small label, I will change gtkrc. However if I correcting font size with gtkrc, the label's font size is changing periodically while the plugin updating it with new values, because of hardcoding panel size dependent 'small' 'x-small' etc. style settings. So the best thing if label font is changeable like in date and time plugin, but if you remove the panel dependent font size changing hack, it will be better than now!
I cannot reproduce your "chaotic mess", though I've never tried to edit things directly in gtkrc. I've simply activated code that's already been there, but maybe setting the font properties via a config option is a better solution anyway.
Fixed in http://git.xfce.org/panel-plugins/xfce4-weather-plugin/commit/?id=dd04f1961900fb7e348b30b748765fe08da001a3. The user can now set the font manually in the configuration dialog, the size is no longer changed automatically. Middle-click on the font button unsets the font and restores the default.