Outside of Chinese class, do people ask 你家有几口人？
Actually, very very seldom.  If (not when) people start asking me question like that, I would ask back: &quot;You FBI?&quot;
As a matter of fact, family information is usually voluntarily revealed, not asked, unless for certain reason, and even that, very specific, not generally.
If you want to get to know a person better, there is only one way:  lots of exchanging information thru chatting.  Family particulars, obviously, are included.  You accumulate this scattered information to form the big picture about the family.