By default GIMP has a checker bord pattern built up of mid tone gray checks. You can however change that in the drop down menu.
That white, black and gray only checks disables checks and the transparency will be shown as a solid black, white or gray color. |
The size of the checks in the pattern that indicates transparency.
If you have set the checker type to white, gray or black only then then the size will have no effect what so ever. |
If you are working on an 8-Bit display, the GdkRGB subsystem of GDK will automatically figure out if it needs to install an own colormap or not. However since GIMP's plug-ins are all separate processes with their own instances of GdkRGB if may happen that GIMP and some of it's Plug-Ins choose different colormaps, resulting in color flickering.
If you encounter such behaviour, you can activate "Install Colormap" in order to force GIMP and it's Plug-Ins to use the same colormap.
If you are working on an 8-Bit display, you can display the "Marching Ants" which show the current selection by means of a rotating colormap instead of re-drawing them all the time.