In 1819 Constable showed the first of his "six footers", the White Horse at the Academy. He was in his early forties and was unhappy with some of the places his paintings had hung in previous exhibitions. In those days the halls were hung with the walls covered with paintings from floor to cornice. Paintings were often "skied" that is hung so high off the floor that they were nearly impossible to see. It was very important to get your painting hing "on the line" where it could be seen. Really big paintings were much less likely to get skied.
Constable was ambitious and wanted to vastly increase his profile at the Academy. He did it by making enormous landscapes. Others had shown large landscapes before, Turner for one. Constable wanted to make showstoppers. The first of these yearly projects was "The White Horse.
There has been a great deal of interest and scholarship surrounding how these paintings were made. Constable did drawings and small pochade (pronounced "pochade") sketches from nature first. He was not the first painter to make plein air studies by any means. I think rather too much is made of the fact that he did little outdoor sketches, but I will come back to that. He also made pencil drawings. Below is one of those, for the White Horse.
He next did something very unusual though. He made a full sized sketch. A roughly painted trial run of the actual painting. He in effect, made the entire painting twice. Below is the full sized sketch of the White Horse. It is the same size as the finished picture at the top of the page.