Set Maximum no of characters using RegularExpression.

<asp:TextBox ID="txtMaximumCharacters" runat="server" Height="105px"
TextMode="MultiLine" Width="342px"></asp:TextBox>
<asp:RegularExpressionValidator ID="RegularExpressionValidator1" runat="server"
ControlToValidate="txtMaximumCharacters"
ErrorMessage="Only 500 characters are allowed."
ValidationExpression="[\s\S]{1,500}"></asp:RegularExpressionValidator>