Just guessing but rats (the unfamiliar scent of) has been known to "scare" snakes unfamiliar with eating them. That would explain the hiss & refusal, much more than the size. Snakes actually don't assess the size of prey very well at all- that's why I'd say it was the rat scent, most likely. As for used mouse litter, try asking a pet store if they'd save you some- & tell them why so they don't think you're totally, lol.
When offering "dead" prey, it may take a little practice to get just the right "presentation"- both warmth, & slight wiggle (using tongs) but not so much that it seems threatening. Remember that in the wild, rodents do not approach snakes & offer themselves as dinner, so try to entice the snake to the prey, rather than approaching the snake with it- which would be quite "forward" & something a rodent would never do. This may seem trivial, but with prey that smells unfamiliar, it matters.