Not that odd of a question, really.
In fact, you ought to get points for originality. Where's Illy Dilly's point tally? Someone! Anyone!
You should each have your own detergent bottle in the 75-78 oz. range, minimum. Bigger might be better, although you still gotta dump it out once per day so it doesn't start to assert itself. I can't remember for sure how many occasions fit in one bottle. At least 2. Possible more. But I don't know how your volume may vary from mine or when you're consciously very well-hydrated like you are on the playa; this is why it's good to have, say: a red bottle for you, and the blue bottle for her and individual ownership, Of course you could share, if you absolutely had to, but it's better not to--that way, there's no dithering in the night and in the dark and shining a flashlight inside it, wondering if you have to take a walk because someone else has taken up the space. (Fishy's right, practicing at home might tell you something if not everything about volume.) If your girl hasn't used a funnel and bottle combo before, encourage her to practice at home anyway
, because not having done so can cause shy bladder (I couldn't bring myself to use a pee funnel for all of 2009, even though I actually had one that year). It takes a little practice. Having the t.p. pre-ripped helps, as does standing like a man.
I can't believe I post this stuff on the internetz.