Yup - we went there too. The Gate of Heavenly Peace was one of the places we visited on our 12hour layover in China in November. It's a big thing, being 66 metres (217 ft) long, 37 metres (121 ft) wide and 32 metres (105 ft) high, and serving as the gateway to the Imperial City, which itself contains the Forbidden City. These days it tends to be used a a saluting podium for offical events - you can see the enormous ranges of seating for the generals either side of the actual gate.
Tiananmen at night (with added tourists)
Of course, Tianenmen is probably more famous for its square, first built in 1651 and enlarged to four times its original size in the 1950s. After another rebuild in 1976 it is now 44.05 hectares (109 acres) in area and is claimed to hold 600,000 people. It's one of the biggest monumental squares in the world, and surround by monumental buildings, including the Great Hall of the People, The Monument to the People's Heroes, the Mausoleum of Mao Zedong, and the National Musem of China.
The Monument to the People's Heroes and the Mausoleum of Mao Zedong
Because it was night, and because we're just not well connected enough to get high above the square to take an overall photo, you will just have to take our word (and everyone else's word too) about just how BIG it is. It's big! Luckily, being a winter's evening it wasn't full so we were able to appreciate the acreage (and having walked one side, the length) but although it felt kind of empty, we weren't the only people there having our photos taken.