Well, that really depends on how you define churchmouse. If you're talking about a mouse that is in a church, then there is no way of directly telling because mice are invincible while on holy ground (it was part of that deal between Reepicheep and Aslan; the tail was just a bonus). You can logically infer from the size of the mouse, however, that it ranges from about five milliliters to about twenty, depending on age. If you're talking about a member or descendant of the Churchmouse clan from Redwall, however, that's a whole different story. That universe has a kind of inconsistent measuring system. On the one hand, fish, trees, and such are much larger than the mice; on the other hand, when they refer to "inches," it's to the mice approximately what an inch to humans would be. So that implies that the mice, and everything else, are friggin' huge, man! So for this breed of Churchmouse, you would either have a similar estimate, or one probably approximating the amount of blood a human has, i.e. approximately eight pints for an adult, about two for a wee baby.
Why didn't they just have the Haitian erase Sylar's memory when they captured him?
Love may be blind, but lust has x-ray vision.
Avatar shamelessly plucked from PMOG.
Also, it turns out I'm not dead--I'm just a right bastard who disappears from the internet for months at a time every so often.