Water bottles are one of those things you simply have to have when you take up cycling. (Unless you’re brave enough to endure the scorn of the “real” roadies and use a CamelBak instead. See my Thought of the Week post on the subject for more info.) Bottles typically come in two sizes: 750 ml (roughly 25 oz) or 500 ml (roughly 17 oz). They also come in two basic types: insulated and uninsulated.
You typically will use about one small bottle of water per hour on a warm day – a lot more on hot days and somewhat less on cool days, so plan accordingly. Longer rides typically require pit stops for more water. In Arizona, for example, I frequently use more than one large bottle per hour on hot days, so I have to stop if I’m going more than a couple hours!
SPECIAL NOTE: You’ll likely want to add electrolytes and/or carbs to your water bottle. See the section on Nutrition for more information.
I definitely recommend insulated bottles over the regular kind. They keep your water cool longer, and having cold water is very important to me. I don’t know the science behind it, but I definitely perform better if my body core stays cooler. Another thing I highly recommend is the CamelBak bottle top. They have a variable-flow valve on top that allows you to simply squeeze the water into your mouth. No other operation is required. All other bottle tops require you to pull the top up first, which opens the valve, before you can get a drink. Inevitably you accidentally hit the top with your teeth, closing it again, which can be very distracting while you’re trying to pedal! Plus, I can never seem to get the flow going as strong as I’d like with this type of bottle. I therefore only use CamelBak tops these days. (Note that I discovered I can put a CamelBak top on my other bottles, converting them instantly to high-performance bottles as well!)