Parallel FP Box Fan Sizing

[enerGwizz]

I'm working on redesigning a tenant space that has existing parallel fan-powered boxes with electric strip reheat. I need to pick out some new boxes... What is a good way to pick the fan size for the boxes? I pick out the box size based on cfm required for cooling, and then there are 3 different options for fan sizes for the box. I'm looking for some input on what works the best. I'm in Southern California also, so the heating season is pretty short, and not too demanding.

An Enviro-tec rep told me they usually size the fan for half the cfm of the supply, but I was hoping for some input from people with more practical knowledge.

Thanks in advance

[JRINJAX]

We use the load calcs on the space to find the neck size for cooling CFM and and then size the heater for the heat loss.
Most boxes have speed selectors on the internal fan to allow different heat strips to be selected.