Some laws can be bent, and some laws can be broken, and some laws can not.
Laws of Physics are some that can not be broken or even bent.
Sound consists of wave lengths, shorts waves are the higher frequencies, longer waves the low frequencies, the longer the wave the lower the frequency, the shorter the wave the higher the frequency, frequency actually means time between waves, how frequent are the waves per second, 1 hertz(Hz) is 1 cycle per second.
Pretty straight forward.
Human ear can generally detect sound vibrations from 20 Hz to 20,000 Hz(20kHz)
If you have a small stick and put it in a pond, how long a wave could you make?
How about a boat paddle?
So that's speaker size and the issue you face
To do high frequencies you need a speaker that is smaller and can move fast, to generate very short waves, 3/4" to 1.5" 'tweeter' is usually used.
4"-6" speaker is used for mid-range sound, it can generate short to long waves but not very short or very long.
10" speaker is about the smallest you "should use", but there are some 8" "subs" available.
The bigger the speaker the lower the frequency you can get, these are the frequencies you feel not just hear.
These are under the seat 'subs': compact car subwoofer at Crutchfield.com
Now about watts, you do NOT need high watts.
Watts work this way
20 watts is NOT twice as loud as 10watts, it is 1/10 as loud, 100watts is twice as loud as 10watts.
So if you have a 30watt sound system and wanted it twice as loud you would need to go to a 300watt system.
In a vehicle 100watts is too much but with pricing these days it is fine, 20-40watts is more than enough to make your ears bleed if that's the effect you want, lol.
So save your money and buy quality not watts.
Subwoofers do need to generate those longer waves, so I would go no lower than 150watt peak rating, and make sure it is a Power subwoofer, it will have the correct amp and a built-in crossover