I'm not aware of a standard size. the 7.5MM base worked out well for me, and we were lucky enough to find one on here.
When we hand-cut our pieces, they were just under 1" square. Thus the 22.5MM frame was a flawless transition.
There should be plenty of packaging to fit your piece.
-a word on ordering candy cups from my experience. pay attention to the base size and the # cup. Make sure they match. I ordered a case of candy cups that were "Supposed" to be exactly what I needed. However when they came, they were a different size. The seller's PDF showing the different cups available had a space in the wrong location. So, the size was actually a LOT smaller than anticipated. The candy cup number i.e. #4, #5... was correct, but it had the wrong description. The seller worked with me to correct the problem, and offered me a discount on another case of candy cups that were the right size, but I still have a large case of very small candy cups... Just sayin, read the label on the case before you open it...
Oh the things you learn.