Some of you might point out that they already have the off-menu 10-oz "Short," but that only applies to hot beverages, so you can consider "Mini" to be the cold version of "Short."
The Mini Frappuccino is being testing in Houston, TX and Denver, CO with a potential for a national rollout next summer if it does well. The cup size is notably narrower than the next size over.
It seems a lot of work to put in for a 2-ounce smaller drink (the Tall is 12 ounces). They did offer a 10-oz cold drink previously in the discontinued Pinkberry collaboration.
Photo via Starbucks.