You will need to set a default value formula on the field that the drop-down list box is bound to, which uses the MAX function. Point the MAX function at the list of values returned from your SP list. Be sure to clear the check box for updating the value when the formula is recalcualted.
The above solution will require that your SP list is queried on load. If not, you will need to write code or use a rule to set the max value when your SP list is queried.