You can have them fall out of the collection automatically after a certain number of days. We want to make sure the customers are seeing the products that you want to display, which are ready for them to order. So new products are not automatically added to collections when you create them, but you can feel free to assign them to a What's New collection as you are creating them. You can also create a product template that you use for any new product, that includes the sizes you typically use, the collections you want to add things to, etc.

Edit your collection to turn on this feature:

"Auto Remove Products after X days (optional)"

You can learn more about Product templates here: https://learn.commentsold.com/ptemplates