Yeah I tried my go at fixing it. A lot of older pre-2.0 mods are like that, the ones that never got fixed. I think 2.0 they changed the size a little of that screen.

From my go at it, I fixed the screen, but the color dots spread out and I couldn't see them all.

The code is somewhere in the customization.gui in \Stellaris MegaCorp\interface\game_setup
You should be able the find the code area, its called Flag something I think. Good idea is compare it to a mod that has it fixed. That is what I did to try to fix it, the widnow size did fix but the dots weren't resized. I am also not a modder or coder, so someone with those skills should easily fix the issue.

Ill try and fix it after work tomorrow. It should be a simple fix. If not Ill take a look at it on my day off later this week.

Im pushing out the update after I post this.
Here's the fixed editor: https://i.serealia.ca/2019/01/stellaris_QH1LQdJ_2019-01-25_14-11-59.jpg