Yes, the mapping will still be the same.
Although I suggest you just turn the file into a .tga then drop it into your customs folder to keep it at 512 resolution

So lets say you are making a 512 head texture. Could you use a 128 head texture to see if the mapping and other things are even?

Aight thanks. I'll try that out.

Make sure to name the .tga file as
'head'
so it would be
head.tga

So Head.tga and put it into the custom folder?

Yes.
I'd recommend you make a new custom folder and call it something like 'test' then go ingame and /lp 1 test, because yours gets auto updated everytime you start your game, and when your game realizes you don't have that head texture active on the forum it will delete it.

Listen here, noob.

1. Make a a new folder in custom named "test"
2. Save the texture as a .tga file name it "head.tga"
3. Go to SP and do /lp 1 test OR /lp 0 test
4. viola.

You do this after you make the 512x512 head.