How to edit the the picture size of the color swatch?

If the color identification at the front desk is not ⚪

If you want the image size to be equal to the color size, you need to add Custom Style at:

 Admin -> MGS -> Theme Setting -> Custom Style: 

.swatch-attribute .swatch-option.image 

{ width: 18px !important; background-position: center; }

And please note that you need to choose the store before adding Custom style, clear cache after changing anything.

Besides, If the color picture is not round, but ellipse: 

Please edit in the  Admin -> MGS -> Theme Setting(English store) -> Custom Style section.

Then, clear the server cache and check your site again to see the changes.