If you call someone a pipsqueak, you are saying they are not worth listening to — too small, too young, or too weak to matter. It is informal, a little comic, and unkind. People use it of a child who shouts orders, or of a junior person who talks as if they run the room. You would not put it in a polite essay or a kind conversation with the person standing there. British and American speakers both have it, though it can sound a touch old-fashioned, like a snap from a story rather than from a careful meeting. The picture in the word is a tiny creature making a tiny noise.