Each of the items listed in Printful’s product catalog have a size guide—put it up on your storefront so your customers can decide which size works best for them.
Push the size guide as an image for your product. When you add a new product from Printful’s side, in the Details step, tick the box next to Add size guide as an image.
Or follow the product’s size guide and create your own sizing table, or put the measurements in the product’s description manually.