Online shopping clothes size help?

I found a Penguins sweatshirt that I really like. It's called a Big and Tall Hooded fleece. I need some help on figuring out the size. The only sizes listed are XL tall, 2X big, 3x big, 4x big, 5x big, and 6x big. I know nothing about sizes so need to know if any of these sizes would fit me. I normally wear mens medium to large shirts. Not sure if sweatshirts have different sizes or if they only make these sweatshirts for people extra large or bigger. Need some help cause the sweatshirt looks awesome