While Rolex GMT or Submariner vintage models are around the 38mm mark, I've noticed that Omega pieces have historically been much larger at 42mm. I love my vintage Seiko divers for the same reason.

 

I always thought the original Speedmaster case was closer to 39mm, and it was the asymmetric case that pushed it to 42mm. I expect an expert to chime in here soon to set me straight

 

Yes, the original straight-lug Speedmasters were 39mm.

The large size of these watches and others, from the 1960s and earlier, were usually meant for professionals - mostly pilots, but also sports-car racers during the postwar racing boom of the 1950s. I wouldn't say that Omega was ahead of its time, so much as tastes have changed to favor larger watches and so the Speedmaster fits that bill. In the case of Rolex, the earliest Submariners and GMTs were 38mm yes, but upsized to 40mm in the early 1960s for better visibility in the environments they were meant to be used in.