UPDATED:
Starting with version v2013 vol 1 (13.1) the ASPxMemo.MaxLength property is available out-of-the-box.
Please refer to the S91467: ASPxMemo - Support MaxLength report for more information.
This example shows how to limit the text length in the ASPxMemo using the HTML5 maxLength attribute. Note that this approach wouldn't work in old browsers that don't support HTML5.
See also:
KA18718: How to use the HTML5 functionality (e.g. attributes, input types) with DevExpress products