Go to systemsettings, select Colours, click on Edit Scheme and select the Colours tab.

The preview widget at the top of that dialog expands when changing the window size, although its content does not require this. This also takes away useful space from the colour list at the bottom. I first noticed it in Plasma 5.8, but 5.9 is also affected.

In the colour scheme selection dialog the same preview widget uses a fixed size (well nearly, it squishes a little when the window is made small).

→ Change the vertical size policy of the the QStackedWidgets in kcms/colors/scmeditorcolors.ui from Preferred to Fixed.