What data type is the ModelID field? Numeric or Text?
Here is a little trick. Create your SQL code in a string like this:
Code:
Dim mySQL as String
mySQL = "SELECT tblSize.SizeID, tblsize.sizeName FROM tblsize " & _
" WHERE ModelID = " & Nz(Me.cboSizeID) & _
" ORDER BY SizeName"
Then before applying to your record source, you can use a MsgBox to see what it returns, i.e.
Code:
MsgBox mySQL
Me.cboSizeID.RowSource = mySQL
Then you can visually inspect it to see if it looks correct.
If you cannot see anything obvious, try creating a query manually using the exact same SQL code (open up the Query Builder in SQL View, and type the SQL code in), and see if it works.
Normally, what I do is go the other way. First create a working example of my query in Query Builder, view the code in SQL View, and recreate that exact same code in VBA. And I use the MsgBox trick to verify it is returning what I think it should be returning.