At the moment, you cannot customize how a Collection appears when published. Looks are set to publish three photos wide, no matter the page your Collection is embedded within. Collections will always load at 100% width, as determined by the page you add the code to. Nine Looks will load at once per Collection. If there are more than nine Looks in a Collection, a “See more” button will appear at the bottom of the Collection. The most recent Look created will appear at the top left corner of your Collection.