MaxLength property of a TextBox control does not work when the TextMode property is set to Multiline. Here is small work around using regualr expression validator..
<asp:TextBox ID="txtComment" runat="server" Width="466px" TextMode ="MultiLine" ></asp:TextBox>
<asp:RegularExpressionValidator ID="RegularExpressionValidator2" runat="server" ControlToValidate="txtComment"
ErrorMessage="Not more than 50 charcters" Display="Static" ValidationExpression='^[\s\S]{0,50}$'>
</asp:RegularExpressionValidator>