It a 6.35 mm. That'd be metric for .25 ACP. For more than that you need to give all the markings on it. I'm suspecting a German copy of the Baby Browning but it may be a .25 auto Mauser too. Try a search for Deutsche Werke.
Some information on Deutsche Werk pistols:
Patented c1916 by Heinrich Ortigies, made by Deutsche-Werke c1922-1932 at Erfurt. Unusual grip safety, only springs back when unlocked with a button on left side of frame above grip, button doubles as a stripping catch. Pressing the button allows the slide to be drawn back, lifted then run forward & off the frame, to remove barrel turn through 90 degrees.
I have a pistol,caliber 7.65mm that has an Ortgies patent built by "Deutsche Werke Werk Erfurt ", Germany. serial Number 262632. It has wooden pistol grips. It has a palm grip safety. I'm trying to figure out history about this particular weapon. Thanks.