How about one with twice (or 3X or ... any integer multiple of your choice)? Carry a spare battery (or 2 or 3) in your pocket.
Why? When that supersize battery runs out, your phone is as dead as when a regular size battery runs out. When I have to switch batteries (the phone gives you plenty of warning, it doesn't just hang up the call) the phone is telling me that I've been talking too much, so I shut up and I still have a spare battery in case I NEED to make or take a call. (You could extend that to the ridiculous - when you put the 8th battery in, it's time to shut up.) If 3 batteries doesn't get you through the day either your phone has something wrong with it or you need surgery to remove the phone from your ear.
You can get 2 normal batteries (and a charger that will charge one battery out of the phone) for $26. One battery that's 3X the charge of the normal one (and doesn't fit in the case, it comes with a special, larger, back) is twice that price. And takes about 4 times as long to charge as a normal size battery, unless you buy a special charger, which is pretty expensive. (So you use the phone all day, turn it off and charge it all night and all the next day? A flat battery can take 24 hours to charge at the rate the phone charges it. It's designed to not overcharge a 2100mA battery.) I can keep my phone running, plugged in, with the spare sitting in the charger getting recharged. Or 2 spares sitting in 2 chargers. (They're cheap.)