I mentioned this a while back, but I usually say small/medium/large when I order at Starbucks. It's my passive aggressive protest against their branding (and also, I feel like the service I get there these days isn't very nice).
If the barista seems really friendly and nice though, I'll use their terminology!
When you order drinks at Starbucks, do you say "tall/grande/venti" or "small/medium/large"?