An on-going problem, not yet fixed.Some are having a 'G'mic screen' with its lower 'bar' off-screen and inaccessible. 'Re-size' arrows do not work. Solution:- when in G'mic toggle the green go/return arrow-tab until you can access and enter the drop-down list for preview size. Click on 'tiny'. Go to Gimp main screen, click on minimize button, then maximize. G'mic screen should now reduce by 30% approx. You can now use 're-size' arrows to adjust screen to your needs. That problem has existed for 3 years now, maybe they'll get around to it one day soon, and it will start with a minimal size which you can then adjust, - simple?. Maybe even get the 're-size' arrows to work in the 1st place?

Which version of Gmic are you using?
Which version of Gimp are you using?
What is your operating system?

I must be one of the lucky ones that hasn't had the issue...

@donnknox
This sounds similar to a problem I was having for a long time. I'm on win7, and one day realized that I had set the screen settings to "medium 125%" for easier reading. Changed them back to default and my problem went away. That is except for I can't read anything anymore. Guess it is time to get glasses.