Update: Sadly... if you put into the Overflow Menu that's expected. Sadly Firefox blocks the size of the pop-up there to fit that specific menu width. I know it's stupid but it's not something I can do... it's the say Mozilla decided to implement it and there's simply no way to do anything there. Just make sure that the addon button is pinned to the addon toolbar (right after the URL bar) and not in the overflow menu. The same goes for max size... it's 800 by 600 pixels at most ... just because that's the hard limit Mozilla imposes for addons, not because I like it, and as well... there's simply nothing I can do about it.

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There should be no problem at all with Firefox 63. I have Firefox 63 on multiple devices and can't seem to reproduce this. Of course, there can be multiple causes for this but most probably it's not the addon fault (sometimes the profile can be at fault, though I really don't know what exactly might cause this).
Have you tried changing the size from the config as well (Config-> Options -> UI Scaling) or just using the resize handle ? If you still encounter problems and have any more details better contact me directly (Options->Feedback, it should open the mail).
Thanks for the feedback.