In the U.S., there are only three sizes for Poodles: Toys, Miniatures, and Standards. "Moyen" is not a recognized size in the U.S.; it is recognized in Europe.
The breed standard for the three sizes is based on the height from the floor to the withers ( the point between the shoulder blades on the dog's back, or the tallest point on the body when the dog is standing on all fours) and are as follows:
Toys: under 10"
Miniatures: 10" to 15"
Standards: over 15"
For conformation purposes, a dog must meet these height standards to be eligible to compete. However, dogs who do not meet the breed standard for height can still compete and earn titles in performance: agility, obedience, hunting, and all of the other competitive activities.
There is no really no such thing as a "teacup" Poodle, a "tiny Toy" Poodle, a "Royal Standard" Poodle, etc. These are marketing ploys used by BYBs. When you see a breeder using these terms, know that you are not dealing with a reputable breeder.
Toys and Minis can go "over size", but they are still Toys or Minis. A 16" Poodle from two Miniature Poodle parents is still a Miniature Poodle, not a small Standard Poodle.
It's also important to know that certain genetic diseases run in some Poodle sizes but not in others, or are more common in one size than another. For example, Patellar luxation is common in Miniatures but not in Standards.Sebaceous adenitis is much more common in Standards than in minis.
It is not a great idea to breed one size Poodle to another, and reputable breeders do not do this. Even in countries where Moyen is a recognized fourth size, they are not "created" by breeding a Miniature Poodle to a Standard Poodle, at least by anyone who knows what they are doing.
In the U.S., only solid color Poodles are eligible for AKC conformation competition. In other countries, they do permit parti Poodles to compete.
And there is no such thing as a purebred merle Poodle. If you see one, know for an absolute fact that some other breed(s) have been introduced into the lines to produce them.