Here are few size comparisons of the new Pentax K-1 camera with other full frame models from different manufactures and the K-3:
Pentax K-1 vs Nikon D610
Pentax K-1 vs. Nikon D750
Pentax K-1 vs. Nikon D810
Pentax K-1 vs. Canon EOS 6D
Pentax K-1 vs Canon EOS 5D
Pentax K-1 vs. Pentax K-3
Pentax K-1 vs. Sony a7
Pentax K-1 vs. Leica M 262
Via Camerasize