The hawk's eye view works for me.

I lay down said prey item and look at it from above. I then look at said snake from above. If the prey item looks the same size as or just a wee bit smaller than the snake, that's good.

If said prey is obviously smaller than the snake, it's time to move up a prey size. If the prey is obviously bigger than said snake, it's time to step down a prey size. Simple.

Gale