First of all, it's a 512x512 texture.
It depends on what head texture item you're applying the head texture to.
If you have a 128x128 head texture, apply a 128x128 texture.
If you have a 256x256 head texture, apply a 256x256 texture.
If you have a 512x512 head texture, apply a 512x512 texture.
Simple.
You could always mix 'n match the textures with the texture items, but it will appear in very (excuse my language, but it's true) shitty quality.
I.E. Putting a 512x512 texture to a 128x128 head texture will appear to be in very bad quality.