Checkboxes – limit # that can be checked

I am creating a form and I want to limit the number of options people can choose (they need to choose exactly 5 options).

I found this:

$("#bla :checkbox").click(function() {
    if($("#bla :checkbox:checked").length >= 5) {
        $("#bla :checkbox:not(:checked)").attr("disabled", "disabled");
    } else {
        $("#bla :checkbox").attr("disabled", "");
    }
});

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however I am unsure where to exactly put/edit this. Any assistance would be greatly appreciated!

http://wordpress.org/extend/plugins/contact-form-7/