What are the minimum and optimum dimensions for the closet for an air handler for a heat pump HVAC system. This is for a 2200 sq ft residence.

Do you know the manufacturer and model number? The equipment you are going to put in there should have clearances listed on the technical information, I would contact them as there may also be requirements for servicing. They typically have a room layout diagram.

I'm in the process of remodeling an older home that does not have a central HVAC system. I'm turning a central hall into closet space so I have centrally located space and space for ducts for 90% of floor space by opening and lowering the 9.5 ft ceilings in the former hall.

I want to provide adequate space for a system. I'm not ready to start talking with vendors and in any case I'm not going to customize for a particular brand or model.

I'm looking for general guidelines. Despite a lot of effort, I've not been able to find useful info on the net. I have found the dimensions of different models of 2 brands. What I'm not clear about is the space needed for service access and for duct work in the closet itself.
Thanks.

Your local code officer should be able to tell you pretty quick. But I bet he says as long as you can gain access to the system to be serviced and or removed with out removing structure from the home. All brand come in different shape and sizes. Remember local installation laws supersede manufacture recommendation, That's the reason they say contact your local inspector for the laws in your area in the back of the installation manual.