Under EditorControl contains the MaxLength property:

<ig:TextEditorProvider ID="WebDataGrid1_TextEditorProvider2">
 <EditorControl ClientIDMode="Predictable" MaxLength="10">
 </EditorControl>
</ig:TextEditorProvider>

This will set a maximum length of 10 characters.

Within a specific behavior can be set under ColumnSettings:

<ColumnSettings>
 <ig:EditingColumnSetting ColumnKey="Message" EditorID="WebDataGrid1_TextEditorProvider2" />
</ColumnSettings>

Be sure to set the column key to attach with the editor id. In this example sets the Message column for the editor.