I know this is too late, but I will post it anyway, as this may help someone else.
I have not tried the size solution presented above, but so I did.
After adding your elements to the container, find the elements js-flickity, see if you can get the data of the object using the method data, and if it undefined initializes the flickering of this element.
var nodeList = document.querySelectorAll('.js-flickity');
for (var i = 0, t = nodeList.length; i < t; i++) {
var flkty = Flickity.data( nodeList[i] );
if (!flkty) {
new Flickity(nodeList[i]);
}
}