I looked into Pico 8 doc and there is a limit on Code size image png or png file for the code, it is 15,360 bytes. The number of lines for the code is 8,921. SO to answer your question of how big your game can be, i say somewhere between NES using the same sprites and gameboy games that last long like pokemon.
Nah I must have miswrote that, you're the second person who misunderstood what I mean't lol. So, yes individual cart has that small of a size. But you can release 16 carts together as a package. I want to write a source cart with all the assets and codes that form the base of the game, then copy that into separate carts and build around that, treating every cart outside the source cart as an individual level.