Comment by colechristensen

Well the ratio of the strong force, vs electromagnetism and the speed of light define the size of the atom. Life requires machinery to self replicate and the distance between a DNA base pair is a sugar molecule attached in a chain so that's about as small as possible. Intelligence requires a certain amount of complexity of something like a brain, and it has to be made of cells and doubtful it could be made more than an order of magnitude smaller.

Could intelligent life exist based on some other physical phenomena than a self-replicating string of atoms? Maybe some unknown quantum phenomena inside neutron stars or something big and slow on galactic scales or something new which fills the dark matter gap...

But otherwise it's physics driving where units of "stuff" can exist, and the correct scales for long term complexity/turbulence can happen, like the thin film of goo on the outside of the frozen crust of a molten rock we are.