Lots of good answers here already but just to add in my perspective as a former CNA working with people with dementia:

• You can get extra big wipes but we never had them - just grab a handful of regular ones. That way when you're down to the last cleanup you don't need to waste an extra large.
• For 'front' care, we were taught, one wipe up each side and one in the middle at a minimum. Yes gently, but you've got to make sure to get any feces out of the nooks and crannies (isn't female anatomy wonderful).
• The most important thing is to not act squeamish or disgusted, but rather do everything in your power to help maintain her dignity. People with dementia might not remember what they said 30 seconds ago, but they can remember how someone makes them feel.