Sizing a mechdeck is mostly about clearance, not the unit itself. Get the clearance rules right and the footprint falls out.
Start with the unit footprint + service clearance
Take the AHU or package unit footprint from the schedule. Add the manufacturer-nominated service clearance on the coil-access side (usually the width of the coil), 600 mm on the other three sides for walkaround access, and a working zone for the electrician on the control panel side.
Add walkway between units
Multiple units? 750 mm between units at minimum, more if the maintainer needs to lift a coil out sideways.
Extend to the compliant edge
The platform edge is where the guardrail lives. Extend the deck out at least 150 mm past the walkway so the guardrail post has structure to fix into.
Verify load path
The steel under each unit needs to land on primary structure. If your deck footprint puts a unit between primary members, you'll either need transfer steel or a bigger deck to relocate the unit.