I got to handle one today along with a K1 and a 645z. It was pre-production, so they weren't allowing sample photos to leave the store. I did take a few photos with my phone for size comparison. Sorry for the perspective, I don't usually take pictures with my phone.
KP w/ 40mm 2.8 ltd left ----- My K5ii w/ 70 2.4 ltd right
and here's one with their big brother, the K1
---------- Post added 02-16-17 at 04:02 PM ----------
So impressions:
The KP didn't feel all that much smaller than my K5ii. The K1 didn't feel all that much larger than my K5ii, though both clearly are. I think you'd have to carry one around for a day or a week or pack for a trip before you really appreciated the size difference. (At least based on my experience today)
The KP is well constructed, just like the other two cameras in this shot. I will say this about it. One of the things that I was reminded of holding it, though is what drew me to the K5. I like that the 3 corners of the exposure triangle are all clear and accessible with external controls and without the screen. Because there is no top LCD and because the controls are so customization, they aren't marked and I felt confused holding it as to how to start my process. I'm sure if I owned it I could set it up exactly the way I wanted and never think about it again, but it didn't like home holding it. So, retro aesthetic, but not in that respect retro sensibilities.
Probably something that would just require a little adjustment for me and then never be a problem, but I kept picking up the K1 which wasn't really on my mind before.