Hmmm....I found this much in the code, but you might already know this:
The table definition for the customer form is controlled by the first few lines of shop$colors.asp with the following definitions:
const TableDef="<center><table border=0 cellpadding=1 cellspacing=1>"
const TableRow="<tr bgcolor='#E0EBE0'>"
const TableColumn="<td><font face='Tahoma, Verdana' size=2 color='#000000'>"
....but I don't see anything in there about the size. Is the size just defaulting based on the other stuff on the page?