It's not as big as you might think. On the east side is the National Museum of China (now closed for renovations) from end to end, and an equivalent size on the west side for the Chinese Parliament (Great Hall of the People). Consider the US capitol---it's about the same width on each side as the Capitol. It looks a bit bigger in pictures due to the six-lane boulevards that surround it. The space is really designed to look big, what with the enormous picture of Mao hung on the Gate (mén) of Tian' An, the entrance to the Forbidden City from which the square gets its name---it's at the north end of the square across the boulevard. The upward columns of the buildings and the Mao-soleum also make it look very big on TV.
I was told (by someone non-Chinese) that most of the actual carnage (hi Firewall!) happened away from the square itself.
It's actually hard to think of it as a place of political agitation and state violence when you're there. Conspicuous military presences in Beijing are now considered rather gauche, although they do have what I'm told is an impressive flag-raising ceremony at dawn. The square is full of tourists (most tourists in Beijing are Chinese), and it has several ice cream and drink stands. Families wander around with expensive digital cameras and then head into one of the two subway stops at the Square. Some of them even picnic there---the idea of picnicking on concrete seems very uncomfortable to me but they were doing it. A couple of guards here and there, and a long queue to enter the Mao-soleum.
I saw the Great Hall of the People this morning, and the Peking opera in the evening. I wasn't able to get into the Mao-soleum on account of not knowing where the bag check was. I'm leaving pretty soon now, and I don't have much time tomorrow, and still haven't eaten Peking duck. I probably won't get to the Temple of Heaven. Oh well, I was here for only a relatively short time anyway, and was busy for most of the past few days.