When the broomhandle came out, it was offered in 7.63 mauser and in 9mm mauser, which was essentially the 7.63 round blown straight. I don't have access to exact dimensions, but I suspect they're similar to the 9x23 Winchester or .38 Super.
Browning made their P-35 in .30 Luger for some reason, and these were fun little pistols. I have no trouble believing that the same thing or similar could be obtained from the EAA witness.