Making FUMBBL full screen seems to be a fairly common question and has been known to put some people off playing altogether. I believe this can be avoided in almost all cases so I've had a go at writing a guide that covers the different workarounds. Its Windows only but I think its Windows users that have the most problems.

https://fumbbl.com/note/HimalayaP1C7/Make-FUMBBL-Full-Screen

It seems as though something along these lines would make a good Help page in the wiki.

I would be very grateful for feedback. If you're having these issues, did the above link help? If you've already solved this problem for yourself, does the guide fit in with how you've solved it?

Edit:
I also have the opposite issue when using FUMBBL on my small screen laptop screen, so here's an adapted version of the above note for anyone having problems on small laptop screens:

https://fumbbl.com/note/HimalayaP1C7/FUMBBL-on-small-laptop-screens