The short answer: The closest you can get to about six ounces. We use a 8 oz. Solo. There may be a smaller, better alternative.
The longer answer: First, to paraphrase a crazy, barefooted dude:... A cappuccino is a specific drink, it's not a type of drink. So the first part of the solution is to grasp the idea firm with both hands that there are no ten ounce, twelve ounce, etc. cappuccino. It's not just a drink of thirds, it's about five to seven ounces for a reason... you can finish the drink while it's still hot, it's, as Goldilocks found out, "just right", and it can be finished so that the texture is throughout the cup without continuing to separate. It's the right amount of espresso and the right amount of thick textured velvety milk that can spread happiness and love. Encourage your cappuccino-ers to stay. Those who really are seeking a cappuccino will appreciate it more. Distinguish between those who really want a cappuccino and those who are just throwing the term about, and really just want a foamy/creamy latte. If you want to make someone a extra creamy latte to go, have at it, but education begins by the cup, or by the cappa.