What's the minimum amount of pixels you can have on a custom skin on servers?

As far as I know (dont know if it changed, been a while since I created a Minecraft server) that functionality of blocking transparent skins is not native to Minecraft proper.

So the answer to this question is hard to get. It depends what plugin is used and what that plugin's configuration is.

So sadly my only answer I can give you is try and see. Log into the server and see if your skin is loaded (and if others can see it.)

If others only see Steve or the black skin, go back in photoshop (or your skin editor), color in more pixels, and try again.

So here is how I would test it.

1. Get a friend on the server with you.
2. See if he sees your skin.
3. If not: Because servers/clients may cache your skin, put on a skin that you know works (like just skin that is completely filled with blue), and see when your friend can see that skin. (So when you try the next version, you know it loaded that new version and not just still giving the old version. Putting that 'neutral' skin in between makes you see when the cache has changed. A few logouts/logins may be necessary so the skin will update.)
4. Color in more of the skin you want, and then go back to step 1.
5. Do until your skin passes.