Items Logical Categories
Add New Item
Go to Inventory-Equipment Settings-Items logical categories
Click ADD NEW ITEM button
Enter new Item's data and check TRACK ITEM BY SIZES radio button
Click SAVE button
Select item from list to: Check Details, Edit Item, Enter Quantities...
Edit Item and click SAVE button
Enter Quantity for each size and click SAVE button
Check Out Item
Go to Inventory-Check Out Item