If you are dealing with a list rather than a library, use JuanTrev's answer. It is perfect!
If you are using a library and you a want no code solution to limit multiline length to greater than 255 chars there is a more involved answer:
The notefield control set the limitation when in a doclib the control limits the multiline to 255 chars. :/ You could set the UnlimitedLengthInDocumentLibrary property to true. But then you lose your limits. We need a multipart solution:
First, in the tag for the multiline in your feature, set the ‘UnlimitedLengthInDocumentLibrary’ attribute to TRUE. Second, to set the 1000 character limit in the publishing page layout, change the rendering template for the NoteField as follows:
< !-- Override the rendering template for the field -->
<asp:TextBox id="TextField" TextMode="MultiLine" runat="server"/><br />
Text="Value cannot exceed 1000 characters."
Credit - This answer was presented originally in a blog by BART MCDONOUGH