Once you add a product, go to the Products page.
Search your product and click Edit Inventory.
Select a size.
Click on Add Selling Locations.
Choose a selling location from the dropdown whose inventory you wish to add for the selected size.
Add the quantity in 'Inventory'.
You may keep the same selling price as mentioned in the product, or you may set a different selling price for the location.
Save the details.
If your company has multiple selling locations (stores), add the inventory for each selling location.
If your product is available in multiple sizes, ensure that you add the inventory for each size.