Trenta is Italian for "thirty" which doesn't explain why the drink size is 31 ounces. Then again, an iced Venti (Italian for "twenty") is 24 ounces (although a hot Venti is 20 ounces.).
The Trenta size is currently reserved only for iced drinks so no giant hot coffees for you! ... yet. I'm not sure how a 31-ounce drink fits in with a brand that puts itself out as an affordable luxury. 31-ounce drinks are more commonly associated with value; it's like buying in bulk at Costco. Then again, if a Trenta iced coffee doesn't get you through the day, you might want to check your pulse.
Trenta iced coffees retail for $3.30 while Trenta iced teas go for $2.60.