Pentax cameras have always come in nice, small packages. I have the K5, which is about the same size as a Leica M (not counting the hump). The grip is very nice, but it is slightly too small for my fingers - and I have a small hand. My ring finger easily slips off the bottom, making me wish for the first time for a vertical grip. Comparing the K5 and this picture of with the 31mm Limited, I'd say the Pentax FF is about the size of the Nikon D7100, which is smaller than the Nikon FF cameras.
I have the MZ-S and I agree that it is strikingly beautiful, and very, very fun.
I also agree that the pentaprism hump with all the angles is kind of ugly, but yeah, maybe that means it will be big and bright.
More importantly, I find the handling and button layout of the Pentax Digitals to be most excellent. Almost all the buttons are under the pointer and right thumb, so it is very easy to do everything with a single-hand, something I can't do with my Nikon.
Then there is the Pentax "green button", which takes you back to the metered Shutter Speed/F-stop/ISO, with the best aperture for the lens you are using. This makes it very practical to shoot manual, even in changing light: Hit green to take you to zero; you can make a situational adjustment, say for backlighting; then hit green to go back home.
The weakness in the K5 was poor focus, especially in darker situations. There were complaints, and I sure found it problematic, but I believe Pentax put a special effort into improving this for the later models.
I'm not super-excited about Pentax FF, since I think the APS size is a much better price-quality-size point. To take advantage of FF image quality, you probably need to build up a collection of of Zeiss or Sigma lenses. Or, if I was really doing landscape, then I'd want the 645D or Nikon 810.
I would love to see Pentax succeed, but the camera sales volume is tanking, and mirror-less ILCs are poised to disrupt the DSLR camera market.