You have to define the columns in the XAML also set the AutoGenerateColumns to be False.
<DataGrid x:Name="dataGridListe" VerticalAlignment="Top" AutoGenerateColumns="False" >
<DataGrid.Columns>
<DataGridTextColumn Header="YourProperty1" Binding="{Binding Path=YourProperty1}"/>
<DataGridTextColumn Header="YourProperty2" Binding="{Binding Path=YourProperty2}"/>
....
</DataGrid.Columns>
</DataGrid>
Then, You need to specify the index of the column
DataGridColumn column = dataGridListe.Columns[0];
column.Width = 200;