Hi:
You can set the column width (say you have one column) to wide enough:
<my1:DataGrid x:Name="DataGrid1" Width="500" Height="500" ColumnWidth="500" >
If you're using DataGridTemplateColumn:
<my1:DataGridTemplateColumn Width="500">
If there're multipule columns you can set the width of the last column to a large value.
Regards