During WW II the Wehrmacht used a 25mm pistol. Of course it was originally a flare gun, but they also developed grenades for it, even an anti-tank round capable of penetrating 50mm of armor (under perfect conditions).
I'm not certain but I believe there have been flare gun "pistols" as large as 37mm, so I think a 45mm one would be possible.
but a 45mm "pistol" as a repeater, using standard type ammo? no. A writer confusing terminology? yes...most likely.
One of the more laughable examples of a "writer" (in this case a journalist) doing "basic" research, but still not understanding what he read was a fellow who, wrote that somebody was using a WW I pistol to shoot people when the police informed him that the gun used in several local shootings was a 9mm Luger....
yes, that really happened...