In GridDataControl, Rowheader height is set by using DefaultHeaderRowHeight property. Refer the following code sample for setting the RowHeaderHeight value.
XAML
<syncfusion:GridDataControl x:Name="SyncGrid" AllowDrop="True" AutoPopulateColumns="False" DefaultHeaderRowHeight="100" ItemsSource="{Binding GDCSource}" NotifyPropertyChanges="True" UpdateMode="PropertyChanged" />