These pouches are for the Walther P5 pistol. The P5 takes a single stack, 8 round, 9mm magazine. I don't have a P5 to measure the mag, but the P-38 mag is similar enough that it can be modified to work in the P5 which means it's exterior dimensions are identical. And I do happen to have a P-38 within reach. It's magazine measurements are below.
Length: 125mm
Width: 32mm
Thickness: 12mm