http://reaudio.com/products/woofers_xxx.php
If you look at the second ported box it says 6.9ft3 but says it only needs a vent that is 10" wide x 3" long. That sounds super small! Could I tune this sub at 20Hz?
We have tried to explain this to you before, but you don't get it. You DON NOT get to choose the tuning frequency. The T/S parameters of the driver do. You do not understand at all how to model a driver.
Now for any driver there is an optimal box where the sound quality will be optimal. This f3 and tuning is always higher than an extended bass alignment that does not usually sound as good. The fs of the driver is a big determinant of the bass extension. Usually the optimal box will tune a little higher than fs. Be very suspicious of alignments with F3 a little below fs.
So in answer to your question no, that driver will not get to an f3 of 20 Hz. The fs is 29Hz. Now the smaller of the two boxes will be better. However the driver is not promising as it has a nasty peaked response in a ported alignment.
Now port size. This is another thing you do not understand.
The air in the port is the equivalent of the weight on an oscillating spring.
What you have to pay attention to is keeping the vent air velocity to around 20 m/sec which is about 60 ft/sec. If the vent is too small then vent air velocity is too high and the vent will be noisy. So you keep the port NO larger than you have to, to meet this criteria. If you make the port bigger it will be unnecessarily long and therefore invite port resonances.
Essentially when you pick a driver, it determines how it needs to be loaded and aligned. You have extremely limited choice in the matter. Your job is to find how to optimize the driver, NOT impose your preconceived notions on the driver. This last point is what I really want you to understand, so we do not keep getting these nonsensical posts from you.