So, Scratch really is tiny, then. I know by realistic standards, he would be colossal for a mouse, but I imagine keeping the height 100% realistic would make some really odd visuals. If you were trying to go for a micro/macro kink, I wouldn't be judging for it, but I have a distinct feeling that's not your goal. In general, furry characters tend to be rather unrealistic, but that usually comes with the territory of being... you know... humanoid animals (not real); rabbits don't have paw pads, but there are far worse inaccuracies out there, not to mention people should do what they want when it comes to fantasy.
So, I imagine the antagonists aren't really that big, just bigger than the protagonist?
On a note related to Mezz and Scratch, the idea of them getting romantic is particularly sweet, as I have a soft spot for cute characters kissing and snuggling. I know that sounds absolutely vanilla, but that is the way I am. I am looking forward to what you do with them, but I do understand that some fucked up stuff will happen first; it is "Cruel," after all.