You could make a sprite that saves a random number to a variable. Or more likely, you could include that same dialog that generates the number in the middle of whatever dialog where it's required.
As far as 16x16 sprites would go, it's not possible for the player avatar to be more than a single 8x8 sprite without hacks. (There is a hack for that.) Some people have created vanilla Bitsy games that create an illusion of a larger avatar using exits, but that has limits. You can of course have a game with an 8x8 avatar but with larger, multisprite characters that they interact with.