Minimum Spray Resolution?

I'd like to make my own, but i'd like to know how big it could possibly be without messing up the image.

The best resolution for a spray is 512x512. The file size also needs to be 512 kb or lower.
The higher the resolution the better, but the resolution needs to be a power of 2 if you don't want the image to be stretched.

If it's a still image like a jpeg then it doesn't matter, it'll automatically resize when it loads up.  If it's not square then it'll be squished into a square unless you add alpha channels in photoshop or something and VTFedit

Many thanks. It's going to be transparent...

And at the risk of sounding lame, it's gonna be my name, like i wrote it on a wall.