if deadpool was cut up into tiny little pieces which part would grow back into a full deadpool

This would happen, it's happened before.

he had his head chopped off once, and he didn't grow back he needed to be reconnected with his body

It's been stated that Taskmaster and Bullseye (or someone) were going to cut up Deadpool into tiny pieces, feed him to piranhas, burn the piranhas to a crisp, and then snort the ashes.... and guess what, Deadpool said it wouldn't work and he'd still heal and the person who had hired the two mercs was himself because he was feeling a bit suicidal, but then they realized then they wouldn't get paid and they'd probably die if Deadpool's body parts burst out of their bodies to try to reattach themselves. Moral of the story, Deadpool is hard to put down, and unless you're Thanos or Darkseid, it's best not to try.