Here's a photo of the PX4 full-size next to a PX4 compact, there's also a photo of the two px4's with an old-school Ruger P85 just to give those that are familiar with the Ruger P series a guideline (in addition to the tape measure).
The full-size is pretty much the same size as the P85 but the P85 is a heavier gun (at least it feels like such to me).
The full-size is a .45 acp, the compact is a .40 s&w, and the old Ruger is a 9mmx19mm (a/k/a Parabellum or Luger)