You can change swatch option styles and the size of the swatch from Option Set Styles.
- Open the Option Set that the Swatch option is associated with.
- Go to Style Tab.
- Scroll to Swatch Style section.
- Adjust the Swatch Style including swatch width and height.
If you want to change the size of swatch options for one single swatch but not all of the swatches in an option set see How to change thee style of specific options within an option set