BigBoy had 4 large cylinders,
all the same size. They were 23 3/4" (603mm) diameter by
32" (813mm) in stroke.
Now, some people call BigBoy a Mallet, this is false. Anatole Mallet invented the Mallet looking to build a very powerful engine able to negotiate sharp curves. This worked by linking two "sub engines" with a pin under one boiler. The front cylinders were MUCH larger then the rear cylinders, this is because steam was used twice before being exhausted. The larger cylinders were low pressure cylinders, the smaller one high pressure. Steam passed from one set of cylinders to the other before being exhausted. BigBoy is NOT a mallet, he is what is known as a simple articulad (while a mallet is a complex articulad). He had two sets of cylinders, each the same size, getting their own supply of steam from the boiler. That steam, after being used, was not sent to the other cylinder to be reused, instead, it was simply exhausted.