The Default in Carrara is inches (and miles) which you can chage to metric etc in preferences'

I f you insert a Plain and look at the Motion Tab, you will see the default size is 20inches by 20 inches.  For a Cube- 5x 5 and the Infinite Plain 64 x 64 inches.

Poser models seem to load in at about 15 inches high.

There you have your starting point. You may feel a need to scale things down though to suit large scenes.

bwtr