Can we do it in anything other than cups? Or, can we define how big a cup is? How many ounces? Or how many liters?

Just joined this program and I think it is great!

I agree, I'm not sure how much a cup is exactly

One cup is 8 oz. If you drink a 16 oz bottle of water, that's 2 cups. Also you can convert to liters and whatnot by searching on google 'how many cups in a liter' etc. it would be nice if they offered different measurement options here on MFP but for now those things should help a bit. :-)

Nobody uses cups unless they're cooking. It's such a pain in the ass to have to do it in cups.
I should be able to select the units, without having to switch to metric (btw, when I tried this, it still listed water as in cups).
My Nalgene measures in oz. & mL.
My preference would be Liters, since I'm pretty used to doing liquids in metric, but ounces would at least be better than cups. Yes, I know I can convert this in my head, but why should I have to?

i can't do cups - i spill

Can't we go for litres?

In the food diary it's a picture of a glass with a number on it, make the number be whatever you want,

if you want to make the number on that picture indicates one bottle of yours that you drink from which is 16 ounces... so be it,
if you choose to make the number on that picture of a glass represent 32 ounces... so be it, it's your tracking, track whatever makes it easy for you to track.