Add this function in JavaScript on your page, I stuck it my "functions.js" file I include on the page anyway.
function checkMaxLen(txt,maxLen) {
try{
if(txt.value.length > (maxLen-1)) {
var cont = txt.value;
txt.value = cont.substring(0,(maxLen -1));
return false;
};
}catch(e){
}
}
Then on the textbox use something like this:
<asp:TextBox runat="server" ID="txtComments" CssClass="comment_textbox" Height="75px" TextMode="MultiLine" onkeyup="return checkMaxLen(this,151)"></asp:TextBox>
Notice the function passes the textbox (this) and the maxlength you want, in this case 150, notice I have 151, the function will limit the maxlength minus one... So if you want to limit the textbox with mutiline set to 100, you enter 101, make sense?