Normally concrete is poured on fresh cut earth, nothing back filled, that way very little settling. So concrete doesn't crack, and not sure of the size, but will need an expansion joint once it gets to a certain size.
it is common practice to add sand and at times rock and fill under concrete.
All concrete guys have compact plates as well as some have jumping Jack compactor.
Any skilled concrete guy will lead you in the right direction.
Expansion joints are not the only factor in cracking , rebar , concrete type, weather and thickness all play a part.
The comment about the mix is correct, the mix with mesh embedded also helps with strength and cracking.
