Shoe size depends on the distance between the heel and the longest toe (which is not always the big toe).
Measuring your feet: on a sheet of paper on the floor mark the back of the heel and the end of the longest toe; measure the distance and check the table for reference. Italian shoes utilize measures in centimeters.
N.B. the table is only an indication and does not account for specific features of your feet. If you are between two sizes, take the largest one as well as if your feet measure differently form each other, choose the largest size too. Remember that it's useful to add between 5 and 8 mm to your feet length to get the right size, especially in case of wide feet, hallux valgus etc.