Here are the speakers sizes by Ranger year: Ford Ranger Audio - What Fits
A subwoofer speaker must be large to generate the long waves of a low frequency, the larger the speaker the lower the frequency it can create, this is not related to power although to move a larger speaker does take more power.
A 15" speaker running 20watts will generate lower frequencies than an 8" speak with 200watts, that is just the physics of it, the 8" would be louder, 10 times louder(see below).
A 15"-20" speaker would be silly in a Ranger, but the cabinet the subwoofer is in can help it create lower frequencies, it is a science and you should read up on it.
A 8" speaker can create lower frequencies in the right cabinet.
You will need to decide how much space you want to lose to a subwoofer, up to you.
Some Ranger cabinet ideas here: Ford Ranger Extended | Standard Cab Single Subwoofer Box
These are designed for the space not the speaker.
Just FYI, 20watts is NOT twice as loud as 10watts, it is 1/10th as loud, to get twice as loud you must multiple wattage by 10.
So if you have a 60watt system, to get twice as loud you would need 600watts, so don't spend money on a little more watts, if you had 60watts and went to 200watts I doubt you would notice much of a difference, 300watts would be minimum, 50% louder