I was going to start with 2 colonies in Nationals because they were a good starter and locally the frame size was used by virtually all. I then found one of the LBKA members was packing up, due to poor health, so bought his hives and he mentored me in that first year. So I actually started with four hives (his hives were WBCs).
I quickly realised the WBCs were 'swarm generators' and the Nationals ran out of space too. Can't recall now, but I think I likely bought a 14 x 12 in my second year and then tried OMFs on a deep and the 14 x 12, over the winter. At that time I introduced top bee space as a trial too. Other factors (than brood box volume) affecting my choice of 14 x 12 were cost of new kit and the spherical brood nest shape anticipated in the jumbo format. I certainly did not want perpetual 'brood-and-a-half' after my experience of it on my WBCs!
I chose top bee space, 14 x 12 and OMF as a good combination Some members of my BKA certainly questioned my changes/choices at the time, but some have, of course, followed the OMF and 14 x 12 frame-size paths.
The WBCs were phased out and the original deeps were converted with ekes.
The Dartington was another trial, on my part - but I did go on to make a second as I was impressed with the mangement strategies, and particularly the over-wintering. Still like and use them, which is probably more than I can say for the 'plastic pig' of the beetainer world, the beehaus.
I have now been impressed with jumbo poly nucs (last winter) and will be trialling a 14 x 12 poly this winter, hopefully - as well as a small trial on a slightly different format.
Regards, RAB