スタバではトールを買え（Buy a "tall" cup of coffee at Starbucks）

"スタバではグランデを買え (Buy a "grande" cup of coffee at Starbucks)" is one of best-selling books lately. But, I would say, buy a "tall" cup of coffee at Starbucks: スタバではトールを買え. Why?

Just think about the principle of declining marginal utility. It's not always true to say, the more one gets, the happier one is. No one can drink a gallon of coffee at once. "Grande" size might be able to maximize your total utility, but it is most likely that your incremental utility will be less than the one you can get from "tall" size.

Also, coffee is different from beer or coke. Many would not drink coffee in one gulp but sip slowly. A "venti" or "grande" cup of coffee will get cold unless you drink fast.

Now all you have to do is:
To bring your tumbler with you not only for getting JPY20 off but also for a nice cashier's filling more than you ordered. Hence, it is preferable to bring a "grande" size tumbler for ordering "tall" size. Note: it is also possible that the cashier fills your cup less than the amount of "tall".
So, just cross your fingers and smile at the cashier as much as you can. Good Luck.