execute code AFTER Recaptcha.reload() is finished
function reloadCAP() {
    if($("#recaptcha_widget img").hasClass('largecap')) {
        Recaptcha.reload();
        $("#recaptcha_widget img").addClass('largecap');
        $('#recaptcha_image').css('height', '62px');
    } else {
        Recaptcha.reload();
    }
}

<div id="recaptcha_widget" class="formRow" style="display:none;">
    <span class="f_label">Enter Words Below:</span>
    <input type="text" class="setWidth" id="recaptcha_response_field" name="recaptcha_response_field" />

    <div class="cantread">
        <strong>Can't read this?</strong><br />
        <a href="javascript:reloadCAP()">Get another CAPTCHA</a>
    </div>

    <div id="recaptcha_image"></div> <!-- image loaded into this div -->
        <div class="clear"></div>
        <span class="smalltext">(click to enlarge)</span>

        <br clear="all" />
    </div>
<script type="text/javascript" src="http://api.recaptcha.net/challenge?k=6LfzMMwSAAAAADV6D04jDE6fKwrJ57dXwOEW-vY3&lang=en"></script>

$("#recaptcha_widget img").one('load',function(){
    $("#recaptcha_widget img").addClass('largecap');
    $('#recaptcha_image').css('height', '62px');
});

.largecap img {
  height: whatever;
  width: whatever;
}