The following XAML (based on some stuff I found in the documentation) does not set the size of the header label to 14 px. You can see in the image that the header label ("Dollar Amount") is smaller than the numeric labels, even though it should be the same.
<sf:SfChart.PrimaryAxis>
<sf:NumericalAxis Header="Dollar Amount" FontSize="14">
<sf:NumericalAxis.HeaderStyle>
<sf:LabelStyle FontSize="14"/>
</sf:NumericalAxis.HeaderStyle>
</sf:NumericalAxis>
</sf:SfChart.PrimaryAxis>