My Pentax ME-Super is definitely smaller than my OM-1n and is a great slr that produces beautifully sharp pictures. Another small slr to consider is the Minolta XD-11. It's about the same size as the OM-1 and has too many great features to delve into here. It doesn't have a spot meter but in addition to taking a meter reading when you press the shutter halfway, it takes another reading immediately before the lens is stopped down and the photo is made. It doesn't formally have "P" mode but it does have Aperture Priority as well as Shutter Priority. When in Shutter Priority if you have set the camera at a setting that won't properly expose the picture, the camera will shift to an appropriate setting automatically, in essence the first Program camera, and certainly the first multi-mode camera.
I was wondering if anyone was going to suggest a Minolta Xd-11 and some Rokkor primes. Fairly compact, well featured unit with great handling, good viewfinder, and a beautiful sounding shutter.
Of course the OM series is tiny and good as well...but there's just something about the old Minolta / Leica bodies and primes that I love.