You can do this, (you may have to tweak it a bit to complie and work for you)
ddlName.DataSource = items.Select(item=>item.Name).Distinct().ToList();
ddlName.DataBind();
ddlSize.DataSource = items.Select(item=>item.Size).Distinct().ToList();
ddlSize.DataBind();
ddlPrice.DataSource = items.Select(item=>item.Price).Distinct().ToList();
ddlPrice.DataBind();
And then find the itemID based on the selection of all three dropdown lists.
This is C# and assumes that you have LINQ
Hope this helps.
Edit-- (if no LINQ)
IList<string> names = new List<string>();
foreach (Item item in Items)
if (!names.Contains(item.Name))
names.Add(name);
ddlName.DataSource = names;
ddlName.DataBind();
</string></string>
//Do similar for price and size.
Edit (use SQL commands)
select distinct Name from Item
select distinct Size from Item
select distinct Price from Item