When you create a new library you have to browse to an existing location, and if necessary add a folder using the MC function in the dialogue, and then select the new folder. You can't enter a path directly into the dialogue box.
If Windows 10 is showing the wrong file sizes, MC will show the wrong files sizes. It is just using what Windows tells it to.
The real mystery is why your Auto Import settings would be cleared. They are in the Windows Registry, so if you use a Registry cleaner or similar software, stop it. Windows Registry cleaners are generally a bad thing, even though they can clear up some mess. They can also silently break things for you to discover later on.