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.
I have the same issue and just deleted thread. When asking question about pico8. I think the best thing is to read through all the forum on the pico8 site. That is what I am going to do now do get the understanding of it all as I have come so far on my game but stuck else where. Maybe we can be newbies together and work out what we need to do lol