As a business user, you can create listings with multiple sizes enabling your customers to choose from different sizes on one listing via a drop-down menu.
The drop-down menu is created once the different sizes is entered in the size field - separated by commas. For example, if you wish your customers to choose from small, medium and large, you write: "Small, Medium, Large". When the listing is created, your customers can choose the desired size from a drop down menu.
Unfortunately, the same function is not possible via the color field. If you sell a one size product in different colors, you may benefit from using the size field and simply enter the different colors separated by comma in this field.