My closet grow was 27" x 38", and total vertical space was 6' from the shelf the plants were on up to the ceiling.
I live in Oregon (to give an idea of my ambient temperatures for comparison to where you live), and the 6" ducting through the light hood gets it's air from the room.
That uses the 435cfm fan, and is also good enough for when/if I need to add a medium-sized charcoal canister.
I have a 265cfm squirrel cage fan attached to 4" ducting that draws cool air into the closet directly from the air outside (through a window insert).
That fan is controlled by a thermostatic fan controller set at 80-f, and the fan cycles on & off every 3 to 5 minutes during the winter, and stays on full time in the warm months.
In the Summer, I replace that with a 3" hose from a portable A/C unit (I made a simple manifold out of a box so I can split the cool air into 3 hoses: one for my grow closet, one for the room it's in, and one routed to the master bedroom next to it.)
Some of it will depend on how much room above your light you have to keep the hot air up away from the canopy level of your plants.
If there is no room for the hot air to rise up, it will stay low in the closet and need extra cooling.
That was why mine needed the extra cooling: I only had about 12 inches above the light hood, and I filled it with pillows to help muffle the sound of the air whooshing through the openings of the light hood's ducts.
If your plants are on the ground and not grown excessively tall, and you have full ceiling height, you might be able to get away with less of a fan set up than me.