For many print on demand products we provide product setup templates and guidance information to ensure your images perfectly match the product being ordered.
If you download the template for the product and size that you're creating, it will highlight how much of the image is used as bleed.
Once you're happy, you can then save your image and remove the guide layers before using it to create your products. If you're intending to offer multiple sizes, we would recommend doing this for the largest size you expect to offer, as our print API can then scale the image down for the smaller sizes, saving you repeating these steps for each option offered.