It can be used positively or negatively, and it can be used to show that satisifaction is derived or shocked is apparent after seeing something that is 'big'.
Sometimes the word big is confused with fat. Please consult the examples.
"He is big": whenever you may hears when a person saying this actually means that the person is making a positive comment. If a women says this whe she has looked at something which she finds pleasing, like a mans figure. This statement could also refer to a mans endowment; which a women finds satisfying. So, try to think of the word 'big' when refered to men as not that a man is fat, but rather one who has a well-shaped and/or one that is hung, where there both width and thickness of his form and his endowment which the beholder behinds pleasing. In this example the word 'big' shows an personal interest in an appeasing, satisifying male.
A negative example of 'big':
Alisha: what do you you think of that guy?
Susan: who him, hes big?
Alisha: So what he is big?
Susan: look at him, hes a little on the fat side!
Big and fat are not to be confused, because big is used when speaking of muscle (the figure generally) and endowment.