To clear up a mutual point of confusion:
To the USA: most other countries don’t sell pizza by the slice because their whole pizzas are all personal sized
To others, especially Europe: Americans sell and request pizza by the slice because the typical American whole pizza is over twice the size of a European one and is meant to be shared by the whole table
@[email protected] Italy also has a bonus extra option of take away pizza 'by the cut' (al taglio) which you order by weight - often in hectogram units. So you order 'tre etti' (three hundred grams) and the guy cuts a slice of the right size off a huge rectangular pizza, generally by eye, and generally very accurately. Great for much confusion in shops not used to dealing with tourists. Also genuinely convenient for ordering the amount of food you actually want/need.