Outside to outside in mm [millimeters] 25.4 mm to an inch. They also give the thickness 15 or 25 mm.
Add a fan. Almost all cases have unused moutnting spaces for an additional fan. Exhaust out the rear, draw air in from the front. The mounting for the front fan may only be seen from inside the case. It draws the air in from the bottom between the metal of the case and the plastic front panel.