Short, pudgy, about the length of a AA battery. Actually, if you have a C battery, it's about that size and diameter.
It's basically the Ascended equivalent to our own .45 ACP cartridge. They love it, and don't like budging from tradition.
And it's nasty.
Actual brass works out to 20x35mm in regular nomenclature. Cartridge Overall Length is 50mm for feeding from standard magazines.
Projectiles vary. Hardball, cavitators, high explosives/dual purpose, hollowpoints, and spike rounds with fragmenting jackets. Versatile, no?
There's not a lot of room in that case once the projectile is seated, so I handwave the fact that propellants will be more advanced in a sci-fi setting.
Here are the worldbuilding snippets that didn't make it into the book that describe the cartridge, from the perspective of an interworld…