I have similar issues w/ my 15" laptop with a 3840 x 2160 resolution screen. I got a little help on Gimp Chat - see the thread here: http://gimpchat.com/viewtopic.php?f=8&t=20076, I’m new to the Pixls forum, so sorry if the link doesn’t work.
I’m OK using the mouse, but 1 problem is that the Foreground/Background icon doesn’t scale with the tool icons, so it is very small, and the swap and reset icons are so small that they are almost impossible to locate with a mouse.
Another issue is any tool with a histogram and sliders, like Levels. The 3 icons for the sliders under the histogram are almost invisible, and again, so small that they are difficult to click on.
Handles on tools like Uniform Transform are also very small.
The little icons that appear near the cursor are also very small and hard to read.
The lines denoting selections or brush sizes are very thin, and can’t be seen in many cases, depending on the colors in the image.
Other issues are covered in the Gimp Chat thread. I have an old desktop with a standard HD monitor, and the UI looks fine, but on the laptop, it can be very difficult to see what I’m doing.
I would also be very interested in any solutions.