If you are adding DataGridTextBoxColumn objects to your DataGridTableStyle, then you can set the MaxLength property on the TextBox property of the DataGridTextBoxColumn.
' Add a second column style.
Dim TextCol As DataGridTextBoxColumn
TextCol = New DataGridTextBoxColumn()
TextCol.MappingName = "custName"
TextCol.HeaderText = "Customer Name"
TextCol.Width = 250
TextCol.TextBox.MaxLength = 8 'max 8 chars
ts1.GridColumnStyles.Add(TextCol)