Usually 32oz is a good size, especially if you have to do water purification you have the correct volume for measuring. Though I wouldn't hesitate to go larger. I added a couple of platypus containers to my system which are lighter and fold down small enough to put in your pocket when empty. Can freeze them and they are durable.. http://www.cascadedesigns.com/platypus/bottles-and-storage/category? I have the blackburn cage also.
*Stainless Steel is an option too if you need to boil water.
Personally...if I was going to haul a bottle that large...I'd probably just wear my hydration pack and fill it with 100 oz of water and be done with it...or invest in a frame bag that holds a bladder and allows the drink tube to come out and be at the ready. I can see myself now banging my legs/feet off that thing every pedal stroke for miles and miles and miles. Well...I wouldn't actually because the bottle would end up in the ditch somewhere after just a few miles of that.
You can also use the bottles as a waterproof container for whatever (food, shirt, etc.)
Also pro hiker tip - you can wrap a few layers of duck tape or gorilla repair tape around the bottle and it will still fit in the cage. They you have tape for whatever, like field repair of a tire puncture, or tent or clothes.
