If i make a head 512x512 size to make it quality better, and my texture size is 128x128, Will it Resize and make my head look better?
Or will it stay big?

It will just shrink down to the right size of your head texture.

the quality will drop when resizing to 128
small details may be less noticable and even invisible when scaled down.

oh k,
I'll just stick to 128 till i get 1m TC lawl

1m?! You only need 20k for 512 head.
Btw 512 is really good, you feel the difference when you buy it.

A tip would be
make heads in 512 cause then you can make easier details
and later buy a 512 head

Like what hamza said, you should first make a 512x512 head ahead of time, and then you can buy the 512 head texture item later on.

Mostly what Bendover Said, The quality will drop when it got resized
The only way to make it better is just buy 512x512 Head Texture Items, or just make the Head Starts from 128x128 Size So you don't have to resize it (But 512x512 Head Quality is still much better)