No built in feature that would allow that. Moreover this is something rather complicated to control programatically since you would have to foreseen and manage the many methos of updating a memo field (paste, etc).
Unless you go for a posteriori checking and warning (like checking for Len(Me.Controls("YourMemoControlName).Text) from the before update event of your memo control, and accordingly canceling the event and displaying a warning message ).
For more sophisitcated approach you will find example code at the Stephen Leban's Web site (great resource): Limit Chars Memo
