Since "tall" is the smallest size at Starbucks, you use the phrase "Starbucks Tall" to refer to someone who alleges to be tall, but is, in actuality, small. This could be in response to a friend/other well-intentioned individual who claims that a romantic prospect is "tall," or as a skeptical response to someone who describes himself as "tall" on his online dating profile.
Shelly <reading Jim's match.com profile>: This dude Jim's says he's tall! <Scoffs> Look at him standing next to his ride in this shot, though! <Sarcastically> Um, yeah - he's *Starbucks* Tall!
Shelly's Mom: Now, honey, just go out with Phil once. He's a great guy with a great job. Plus, he's tall.
Shelly: <skeptically> Mom, do you mean "tall" or "Starbucks Tall?"