Rohrbaugh R9
Caliber:9mm
Barrel:2.9in
Length:5.2in
Height:3.7in
Weight:13.5oz
Capacity:6+1 Rounds
Kahr PM9 / CM9
Caliber:9mm
Barrel:3.1in
Length: 5.42in
Height: 4.0in
Slide Width: .90in
Weight: Pistol 14 ounces, Magazine 1.9 ounces
Capacity: 6+1
Kahr MK9
Caliber:9mm
Barrel:3.0in
Length: 5.3in
Height: 4.0in
Slide Width: .90in
Weight: Pistol 22.1 ounces, Magazine 1.9 ounces
Capacity: 6+1
The Boberg is taller height wise but shorter OAL:
Boberg XR9-S (Shorty)
Caliber:9mm
Barrel:3.35in
Length:5.1in
Height:4.2in
Width: .96in
Weight:17.5oz
Capacity:7+1 Rounds
So I think the MK9 is the pistol with the third smallest footprint overall. Not only is the Rohrbaugh smaller but the Boberg as well.
But the others, Diamondback DB9, the Sig P290, Kimber Solo are larger than the MK9 - mostly going by OAL - going off of the manufacturer's specs as stated on their website:
Diamondback DB9
Caliber:9mm
Barrel:3.0in
Length:5.60in - longer than the MK9
Height: 4.0in - same height as MK9
Width:0.80in - thinner than the MK9
Weight:11oz - lighter than the MK9
Capacity:6+1 Rounds
SIG SAUER P290
Caliber: 9mm
Barrel: 2.9in
Length: 5.5in - longer than the MK9
Height: 3.9in - shorter than the MK9
Width: 0.9in - 1.1in w/slide catch (roughly the same width as MK9, Kahr lists the width of the slide only)
Weight: 20.5 ounces w/magazine - lighter than the MK9
Capacity: 6+1
Kimber Solo
Caliber:9mm
Barrel:2.7in
Length:5.5in - longer than the MK9
Height:3.9in - shorter than the MK9
Width: 1.2in - wider than the MK9
Weight:17oz - lighter than the MK9
Capacity:6+1 Rounds
Beretta Nano
Caliber:9mm
Barrel:3.07in
Length: 5.63in
Height: 4.17in
Width: .90in
Weight: 17.67 ounces
Capacity: 6+1
Ruger LC9
Caliber:9mm
Barrel:3.12in
Length: 6.0in
Height: 4.5in
Slide Width: .90in
Weight: 17.10 ounces
Capacity: 7+1
Glock 26
Caliber:9mm
Barrel:3.46in
Length:6.29in
Height:4.17in
Weight:19.75 oz w/o magazine
Capacity:10+1 Rounds