True IMAX screens are absolutely massive and like almost like a square when you're standing in front of them. These are 50-60 feet (15-18m) tall, sometimes even more.

The vast majority of true IMAX theaters are found in museums as IMAX were exclusively used in short documentaries from the first film released in 1970, all the way until The Dark Knight became the first narrative film to use the camera for part of the movie in 2008. Because of how unwieldy the camera is and the extreme expense of the film stock it uses, no feature film has ever used it for its entire run time, and it's unlikely that one ever will.

There are, however, a few full size IMAX theaters out there outside of museums. The IMAX theater is Sydney will be the largest of these, and in the US, there's one in New York in a few in California. Wikipedia's list of IMAX venues denotes which theaters are formatted to the 1.43:1 aspect ratio.