Menu: View→Object Properties→Parameters→Font size
The size of the font used to display parameter values for the currently selected object.
See also:
To change the font for a parameter
Programming
In Amos Graphics programs, you can select the first item in this ComboBox as follows (Visual Basic):
Dim aComboBox As Windows.Forms.ComboBox = pd.GetComboBox("", "ParameterSizeCombo")
aComboBox.SelectedIndex = 0