MTB Sizing. Saddle at the very bottom to reach pedals perfectly. Go one size smaller? So im buying my first MTB. Its a Scott Scale 930. With the size M i get the perfect reach to the pedal with my heel. But only when the saddle is set to the very lowest/bottom. Should i get a size S instead? My height is 171 cm.

Thnx
If you have to put the saddle all the way down, the frame is too small large. You need to have a comfortable clearance between the top tube and your crotch when your feet are flat on the ground - especially on an off-road bike, since you'll often have to dismount in a hurry. There are other aspects to getting a proper fit, but this is something you can't get around.

Also, if you're on the short end of things, I wouldn't recommend a 29er. They do make them in small sizes, but in order to get a smaller rider into a reasonable riding position between those two big wheels, they have to make some compromises with the geometry. You'd be better off with 26" or 27.5" wheels.