The t-shirt canvas size.

Above is a general chart on the size of most adult t-shirts.

the actual area that can be imaged depends on the reproduction technique being used. Most production techniques use a board ( called a Platen) to stabilise the garment, this and other factors such as Squeegee size screen size and Film output will dictate your canvas area/ Belt printers are capable of printing an entire t-shirt seams sleeves and all.

Many large event shirts are printed in two sizes, one for youth and one for adult. Sleeves use a smaller pallet or platen. Consult your printer for exact sizes.