Use the maxlength like the above post or you can use javascript and a customvalidator control to check the length.

Use a custom validator with ValidateTextLength as its clientvalidatefunction name

function ValidateTextLength(source, args)
{


 var length = $get('<%=txtYourTextBox.ClientID %>').value.length;


//this would make it valid unless the length is greater than 10
if (length <= 10)
     args.IsValid = true;
else
     args.IsValid = false;
}