A layered drink is sold twice. Once when the customer orders it, and once when the person at the next table sees it go past. The second sale only happens if the cup gets out of the way.
The cup is part of the drink
Clear PET cups at 16 oz are sized for dirty soda, iced coffee and layered cold drinks where the clear cup is part of the presentation. That last phrase is the whole point. Put the same drink in an opaque cup and you have sold it once instead of twice.
This is the opposite of how dessert cups work. On the frozen side, the cup is a container and the color is branding. On the cold drink side, the cup is a window and any color in it is competing with the product.
Build order matters more than usual
If the visual is doing the selling, then the order in which things go into the cup is a service decision rather than a matter of habit. A drink assembled carelessly muddies immediately and looks like every other cup on the counter.
Whatever your build is, write it down and have everyone follow the same one. The inconsistency shows in a clear cup in a way it never does in a paper one, and a customer who ordered the drink they saw on someone else's table will notice if theirs arrives looking different.
Sizing the range
Sixteen ounces is the standard. Twenty-four is the large, and it comes six hundred to a case rather than a thousand, which tells you something about how the two sizes actually sell.
Below that, 9 oz covers sodas, juices and sampling, and above it 32 oz exists for shared and blended drinks. Two sizes is usually enough here for the same reason it is enough on the dessert side, which we set out in choosing an 8 oz cup. A standard and a large is a real choice. Four sizes is a wall of nearly identical cups that nobody can tell apart at arm's length.
Do not stop at the cup
A cold drink that travels needs a lid, and the lid choice changes what the drink can be.
Flip top lids cover 12, 16 and 20 oz with a hinged tab that opens for drinking and closes again, which suits anything going into a car or a bag. Dome lids leave clearance for whipped toppings and tall builds, and there is a version sized for the 107 mm rim on the largest plastic cups. If the drink has anything sitting above the liquid line, you are in dome territory for the same reasons we went through in dome or flat lids.
Straws are part of the picture
In a clear cup the straw is visible for the entire drink, so it is not a neutral item. Paper straws come in striped patterns, and there are color changing versions that shift as they chill in a cold drink. On a counter already running the color changing spoons, extending the effect to the straw is the obvious move.
The full set of cold cups sits in the cup range, with the lids and straws alongside. Start with one size, one lid and one straw, and only add the second of anything once the first is selling.