Adding this to the adv. stylesheet will allow popups to be resized dynamically. Without this, the size gets stuck when you're trying to reduce the size (I believe). Taken from here.
.ia_popup[id^="popup-"]{
height: auto !important;
}
Also, setting the popup View's props.defaultSize.height: auto will auto-size the popup's height dynamically to its content which is quite useful.