Sorry guys for this question....just started c5p.  Played with it a little bit ..just to get to know all the tools , etc...and did a few step by step tutorials ,etc...
I'm going to work on a very simple, sci-fi looking building. Simple shapes ...should be easy to create it.
Ho do you set up the size for the building (or any real life object....like a car or a street scene or 
a bridge , etc..) ?  When you create a street scene which covers 4-5 city blocks...how do you start...size etc.. ?
Thank you very much

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.

I use actual size measurements in Carrara.  I haven't done anything as large as city blocks though.

shonner
So , if you model a car ...(after a real life car) and it is about 170inches long..then you model with 170 inches for lenght and so on......? and can use the real life measurements for all the parts..components

make it short.......I can model ... between...Couple of inches to actual size .

by the way......thank you guys 

I use inches and sometimes feet.  But mostly inches.  When I do bevels, it's easy to work with .25 - .50" numbers that I'm used to.  Anything Goos is more accurate along edges as well when used with inches.