6" aero will work. more would be better since 6" is right at the vent mach where you actually need the flares of the aero.
as long as you keep the tuning around 33hz, the 6" aero will be fine
3 cu ft vs 5 cu ft won't make much difference (and if you never build both you'll never know)
but, if you have the space for 5 cu ft, you could go 4th order bandpass 2cuft rear & 2.5 cuft @55/hz (6" port 5.5" long)
and be louder/ sound better