Ventilation needs for heat pump water heater in outdoor utility closet

I am working with an HVAC contractor to replace my current gas-power water heater tank with a heat pump water heater. It will be located in an unconditioned utility closet with a door connected to the outside (see photos below). I plan to replace the door also.

The closet itself is quite small, so I need some air ventilation. I wanted to get some other opinions about what sort of door or vent I can get that will be sufficient. Will a door with a small louver work? How big does the louver need to be and where should it be located? Could I just use a vent instead and have a solid door? Any suggestions would be welcomed, thanks.