Set the MaxLength for the Telerik Grid View Data Column

 <telerik:GridViewDataColumn Header="Code"
                                            IsResizable="True"
                                            IsVisible="True"
                                            DataMemberBinding ="{Binding Code,Mode=TwoWay}" 
                                            IsReadOnly="False">

                        <telerik:GridViewDataColumn.CellEditTemplate>
                            <DataTemplate>
                                <TextBox Text="{Binding Code,Mode=TwoWay}" MaxLength="12"/>
                            </DataTemplate>
                        </telerik:GridViewDataColumn.CellEditTemplate>
</telerik:GridViewDataColumn>