image from artrenewal.org
The best known of the six footers is the Hay wain, shown above. I want to talk a little about why one might make a full sized sketch for a painting that size.
I don't find it particularly remarkable that Constable made these sketches. I have done full sized sketches for my own paintings. I did it for a couple of reasons and I assume that Constable did too. You can push a 24 by 30 around forever if things don't go well. A painting the size of the haywain could fight you nearly forever with it's reworking. By the time the battle is completed you may have the image, but it looks fought over. That is, passages have become too thick and there are pentimenti, or the slightly visible ghosts of previous parts of the image, now painted out. Also a painted passage looks best if it is painted authoritatively. That is more likely to happen when you know exactly what you intend to do, and do it, not as the result of reworking over and over again.
My own teacher, R.H. Ives Gammell, did this too, he painted complete versions of his allegorical pictures and then copied them to make the finished image. It seems laborious, and is. But it does solve the problem of a piece looking labored over. After making it once and with the full sized sketch as a guide it is relatively simple to make another copy of that, making small alterations and finessing things as you go.
Today there is so much attention paid to the sketches and I think it misplaced. They ARE interesting as a window into Constables working methods. But that's not why they are so popular. People today like them because they better fit our contemporary preference for looser more "expressive" art. While there is nothing that says you can't like them, I would caution you against missing the great beauty of the finished pieces which are the artists actual intent. The sketches were merely a step on the road to the creation of the finished salon pieces for Constable. They are how he wanted his ideas realized in their final form. Valuing historic paintings based on how much they fit our own times preferences brings with it some dangerous baggage.
When I was an art student, art history was thought to be progressive, and those pieces which "led to the triumph of modernism" were deemed important. Those which did not, were ignored, or like Bouguereau reviled. The idea that the art of our time is transcendent and all art before it matters only as shown by its reflected light is chronocentric ( yes, that is an invented word, I needed one to explain the idea, Oswald Spengler should have, but evidently didn't, perhaps chronological imperialism might be useful too)
For an example, let me turn to rock and roll, as I so often do. Despite the warm charm and prodigious wisdom of Lady Gaga, the Beatles are not better or worse because of the degree that they are her percuser. Their music was good because it was well made, even without it's influence on the delicate slaughterhouse songbird, Lady Gaga. If Lady Gaga proves to be a passing fancy, will that diminish what the Beatles did, will they need to be reappraised as leading to say....Ke$ha?
The Beatles wrote and played some great rock and roll, and had the multitalented meatdressed Gaga been stillborn, that would have still have been the case, despite her great chops.