Since mugs in the past didn't keep coffee hot for long anyway, most hunters used small ones. Now, however, new technology can keep a LOT of coffee hot for a LONG time. Consequently, the sky's the limit when choosing the size of travel mug to purchase. Keep in mind, however, that depending on the size of the drink holders in your vehicle, a big one might be more of a nuisance than a convenience. The same goes for wide-bottom mugs designed to be less likely to spill. If they won't fit in your cup holder, they're actually more likely to spill. Buy as big of mug as you find handy, though, so in the afternoon your coffee mug can morph into an iced tea glass!