Thanks for the screen shots of the table structure, this is very helpful. I'm still a little confused though about how these tables relate to one another.
If you have a unique item per item id, an each item id has a single size id how do you associate multiple sizes with a single item?
Usually in this scenario you would have a relational or joining table. This table would hold item id's and corresponding size ids. So in this table you could have the same item listed multiple times but the sizes entry will be different for each item entry.
This is the part that I'm not making a connection with. If the item table holds a single unique item for each id then you would only ever be able to correspond that item with a single size id.