Q:

winforms window too large

I think the images say enough...
The grey part of the window overlaps the blue side. 
I can fix the problem with the normal window by making it a little bit larger or if I change scaling to DPI.  My laptop is set to 125% scaling, I think that is the problem?  But how do I fix the messagebox? 
I am using Visual studio 2017 15.2 it's an C# Winforms app with .net framework 4.6.2
MessageBox window too large:

Window too large:

A:

This should resolve itself just by adding DpiAware setting to your application.   To do this right click on your project, Add, New Item and then look for Application Manifest File in Visual C# Items. 
Once added open this and uncomment the section containing dpiAware (in Windows settings)
HTH

