server site validation

There can be various ways to do it.

1. Setting MaxLength (already suggested above)
2. Use Javascript to track the characters filled based on onchange/onkeypress events
3. Use Regular Expression Validators to take care
4. Use Masked textboxes (option 2/3 is internally implemented)

Sandeep is correct.
there are various ways to do it.
if you want to validate your user input using javascript heres a basic function

<script type="text/javascript">

function validate() 
{ 
    if (document.getElementById('txt').value.length > 50) 
    { 
        alert("error message"); return false 
    }
}
</script>

<input type="text" id="txt" />
<input button="submit" onclick="return validate();" />