<script>
function check(nazwa) {
var Obj = document.getElementById(nazwa);
var length = Obj.value.length;
if(length != 11) {
alert('Pesel musi miec 11 znakow.');
return false;
}
else return true;
}
</script>

<form method="POST" action="abc.html" onsubmit="return check('a')">
    <p><input type="text" size="20" id="a"></p>
    <p><input type="Submit" value="Submit" name="B1"></p>
</form>