I thought you would have had responses by now, hopefully my comment will stimulate a few.
I have owned Blake Seamasters in both the mid and full size. I much prefer the full size. This model is very slim and has a very organic curved shape so actually wears smaller than its bare 40mm measurement suggests. You mention the ratio of dial to bezel. IMO on the mid size the bezel robs the face of valuable surface area leading to a tiny dial with stumpy hands vs the full size. The mid size actually has a similar dial area to that of a 30mm non diver watch, ie tiny. There is one upside to the mid size, they are far less popular so are available from markets with slimmer wrists (like Japan or Asia) much cheaper then the full size, maybe as much as $300-400 less.
Regarding the dial, the wave pattern on the dial is in relief and can look different in different lights so I wouldn't worry about that, they all look black in the flesh.
In summary, unless your wrist is very small, like 6.5" or smaller I would say that the full size should work. Try one for size and hopefully you'll see what I mean.
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