In design view, click the Select List.
In the properties inspector for the select list, click on the Dynamic button.
In the Dynamic Select List window, the static section allows you to add a static option. Add a static option, set the value to be blank, and the label to "Select A Size".
below that, from the Options From recordset list, select your size recordset.
In both the value and label list, select the Size Name column.
See the enclosed screen shot for an example. Your recordset and columns may have a different name.