Setting maxlength of textbox with JavaScript or jQuery

Not sure what you are trying to accomplish on your first few lines but you can try this:

$(document).ready(function()
{
    $("#ms_num").attr('maxlength','6');
});

The max length property is camel-cased: maxLength

jQuery doesn't come with a maxlength method by default. Also, your document ready function isn't technically correct:

$(document).ready(function () {
    $("#ms_num")[0].maxLength = 6;
    // OR:
    $("#ms_num").attr('maxlength', 6);
    // OR you can use prop if you are using jQuery 1.6+:
    $("#ms_num").prop('maxLength', 6);
});

Also, since you are using jQuery, you can rewrite your code like this (taking advantage of jQuery 1.6+):

$('input').each(function (index) {
    var element = $(this);
    if (index === 1) {
        element.prop('maxLength', 3);
    } else if (element.is(':radio') || element.is(':checkbox')) {
        element.prop('maxLength', 5);
    }
});

$(function() {
    $("#ms_num").prop('maxLength', 6);
});