Recently posted the Opera model from Gruen. The emphasis on thin watches was a Gruen trademark. I did some rough measurements of movements including dials. Opera which is 0 size 4.3mm. The Verithin 5mm. Semithin 5.3mm. Ultra Verithin 4.3 mm. There were 2 small sizes model 716 and 717 marked Gruen Guild also about 4.3mm. Waltham 1898 12 size 6.3mm. Elgin 1928 12 size 6.0mm. The last Verithin design ~1940 also 4.3mm. Gruen Swiss made thin movements were good quality and durable; some Swiss made specials were 1/2 thickness of Gruens.
Photo of 3 here is Veri-Verithin looks like 14 size because of larger diameter thin looking case with Verithin movement. Semithin like American 12 size. The uncommon Gruen Guild looks like a 10 size. Its movement is 34mm diameter compared to Verithin 38mm.