What would you use for in? 4? Or passive?
If your going active intake then one step down, enough to be a bit less than your extraction once you've factored in your losses from duct and filter.
If you go too small your restricting it more than aiding it, but you do have extra passive vents to make up for it.
The way I would calculate the extraction would be watts x 3.16/ DT( temp rise from ambient in f)
So 1500w x 3.16 / 10f (aiming for 74.4f desired temp) = cfm
Which would give you 474cfm without losses.
Really you should add any other wattage , fans ect into that total wattage.
I would be adding 35% at least for duct loss and filter, depends on how many bends you have, type of duct.
I would always want more than less and ideally use a fan that's got a speed controller
Something like an 8" ac infinity should cover you.
Assuming your 18 degrees c is worst case ambient, otherwise use what ever ambient you get in summer time.
Personally I found in a 1.2x2.4 I didn't need an intake