Try something like this:
Code:
Select 
	Items.BarCode ,
	Items.Name ,
	Items.Size
	SUM(Sold.Units)
From 
	Items
INNER JOIN Sold
    ON Items.BarCode = Sold.BarCode
Where Sold.[Date] BETWEEN "Date1# AND #Date2#
(This is a bad field name (DATE) to use by the way as it is a Reserved word)