Shops that already run color changing spoons usually run one size, and usually the wrong one for at least half of what they serve. The effect is identical either way. What differs is whether the spoon fits the cup it is going into.
Two sizes, two jobs
The standard tasting spoon is a sampling utensil. It is small, it takes one bite, and it belongs at the station where undecided customers are working through flavors.
XL color changing spoons run longer than the standard tasting spoon. That length is what makes them a serving spoon rather than a sampling one, and it matters most in a tall cup, where a short spoon disappears below the rim and the customer has to reach in after it.
Match the length to the cup, not to the price
The practical test is your largest serving cup. Stand a spoon in it. If the handle does not clear the rim comfortably, the customer is going to end up with cold fingers and you are going to hear about it.
Small cups have the opposite problem. An XL spoon in a 3 oz or 4 oz cup is unbalanced, will not rest against the side, and tips out when someone sets it down on a table. That is the same fit problem we described in mini tasting spoons vs full-size spoons, and it does not go away because the spoon changes color.
The obvious pairing
Standard at the sampling station, XL in the serving cups. Two lines, one rule, and staff never have to think about it.
The reason this works better than picking one size is that a customer who samples and then buys gets the effect twice, in two different formats, and the second one arrives in the cup they are paying for. If you are only going to stock one, stock the size that matches your standard serving cup rather than the cheaper case.
What the confetti mix is for
The XL comes in five color pairs plus a confetti mix, and the confetti option is doing something different from the others.
A single pair gives you a consistent look, which suits a shop with a settled color identity. Confetti gives you variety within one case, which suits parties, events and children's service where the point is that nobody's spoon matches anybody else's. If you run birthday bookings alongside normal counter trade, holding one pair for everyday and confetti for parties covers both without a second decision on the day.
Cost, plainly
The XL case costs a little more than the standard, and every colorway within each size is the same spoon at the same price. So the only real question is size, not color, and the color choice can be made purely on how you want the counter to look.
Both sizes, the straws, and the heavier weights sit together in the color changing range. If you have not run these before, the mechanics and the pair-choosing logic are set out in how color changing spoons work.