Items Logical Categories
Add New Item
Go to Inventory-Equipment Settings-Items logical categories

Image: Items logical categories
Click ADD NEW ITEM button
Enter new Item's data and check TRACK ITEM BY SIZES radio button
Click SAVE button

Image: Add new Item
Select item from list to: Check Details, Edit Item, Enter Quantities...

Image: Item Details
Edit Item and click SAVE button

Image: Edit Item
Enter Quantity for each size and click SAVE button

Image: Enter Quantities
Check Out Item
Go to Inventory-Check Out Item

Image: Add new Item