The uterus is about the size of a fist when not pregnant or menstruating, but I think you're asking more about the vagina, which is longer than the average human tongue or finger. Consider that a male penis gets inside without damaging anything, and even if it does reach the crevice, there shouldn't be a problem.
Don't stick sharp long objects inside her (dildos and vibrators which are designed for this purpose are ok), and you definitely don't have to worry about tonguing or fingering.
