This approach is now obsolete. Starting with version v2013 vol 1 (13.1) this functionality (the ASPxMemo.MaxLength property) is available out-of-the-box.
Please refer to the S91467: ASPxMemo - Support MaxLength report for more information.
This example demonstrates how to limit the length of text that can be entered into the control using the MaxLength property.
See also:
How to implement the "characters remaining" functionality for ASPxTextBox and ASPxMemo using MaxLength
How to set the text max length in the ASPxMemo control
