Hi,

You can try this snippet:

xmlns:igPrim="http://schemas.infragistics.com/xaml/primitives"
...
<ig:XamGrid x:Name="XGrid">
    <ig:XamGrid.HeaderStyle>
        <Style TargetType="igPrim:HeaderCellControl">
            <Setter Property="FontSize"
                    Value="16" />
        </Style>
    </ig:XamGrid.HeaderStyle>
...

HTH