how do you resize head texture?

Head textures automatically resizes if you have a lower resolution item than the texture itself, so lets say you have made a 512x512 head, and equip it on a 128x128 Head Texture Item, it will be resized, but will loose some quality.

512 is 16 times bigger than 128 so you'll lose 16 times the quality.

How much cost generally a head texture and how to equip it?

You can buy the 128x128 head texture for a simple 200 TC. but the 256x256 and 512x512 are much more expensive.
You can activate them through going to your deactivate list, activating it, then going to your textures and putting whatever head texture you have available~