They are often small, ( the name derives from the French for "pocket" ) although they can now be bought in all sorts of sizes and designs.
I made mine from the remains of an old wardrobe many years ago when pochade boxes were much rarer, but by making my own, I was able to incorporate all the things I thought most important.
I decided that the box should be small so that I could make quick oil sketches and even use it when sitting in a car. So I settled on a painting panel size of 7 x 10 inches. In order to make sure the panels I would be using would fit, I made the lid first and routed out three slots so that the box would take up to three panels.
I then made the storage part of the box to fit.
I like the idea of a sliding palette rather than a lift-out one which some boxes have, which means having to find somewhere to put it.