The right size of sweatshirt for you depends on how you like clothes to fit. If you're just choosing a sweatshirt to lounge in, your usual jacket size will probably be comfortable. If you are buying a trendy hoodie or pullover that's designed to flatter your figure, try your normal shirt size.
A laundry tip: Washing can alter sizes. Read the labels; a sweatshirt that is 100% cotton will probably shrink if you dry it in the dryer. If you want to avoid shrinkage, hang sweatshirts on a plastic hanger from a hook or shower rod. Hanging sweatshirts on a hanger allows the air to circulate and they will dry more quickly than they would if they were draped over a drying rack.