Make my Shopify website require customers to choose a size before they add an item to their cart

I'm looking to add a feature to my Shopify website that requires customers to select a size before they can add an item to their cart. I want to use the size chart option to input the sizes, which will allow customers to quickly select the correct size for their purchase. I also have a custom size option already installed. If the customer selects a custom size I would like that sizing information to appear along with the order rather than be emailed to me separately.