Room by room load calc, Which will give you the CFM you need per room. Then you can use manual D to determine what FR to use to size your duct work, and manual t to determine what grilles and registers to use.
You must start with knowing how much heat the house is expected to gain on what's called a "design day" for your region. Get a copy of Manual J and read through it. Manual D for duct design. Manual S for equipment selection methods.
This is not something you just "wing it" on and hope for the best. Do it right or hire someone who can. Especially in California, with duct leakage testing and Title 24.
Electricity makes refrigeration happen.
Refrigeration makes the HVAC psychrometric process happen.
HVAC pyschrometrics is what makes indoor human comfort happen...IF the ducts AND the building envelope cooperate.
A building is NOT beautiful unless it is also comfortable.
Not a pro, so please don't take this as technical advice:
If you are trying to do this yourself, it can be a very, very long road and although it may not seem like it now, winter is coming. This site doesn't allow DIY, so you won't get much real help here, anyway. Be careful, from what I seen, the "other" forums geared toward HVAC & DIY contain a lot of confusion and dis-information... call a pro
If you just want to understand what your contractor is proposing/doing, then reading manual J and D sounds like a good start. They're not cheap, though.