Mouth movement (mouthing) is part of facial grammar in sign language. It conveys an adjective, adverb, or another descriptive meaning in association with an ASL word.
Three major mouth morphemes which describe sizes are: "oo", "mm", "cha". Let's quickly look at these meanings:
The mouth morpheme "CHA" represents something very big, very large, gigantic, colossal, very tall, huge, etc. The eyes tend to be wide.
The morpheme "MM" conveys a meaning for medium, average, regular, middle, moderate, etc. The eyes and closed lips are usually relaxing. Not everything in signing with normal closed lips means that. It's contextual.
The round mouth "OO" means: tiny, small. With the pursed lips, the eyes are typically squinted. The shoulders that shrink slightly also can be used sometimes to convey "very very tiny or small", for example.
Below shows a few examples of mouthing which represents a size of a book. Facial grammar is integrated with a classifier (a handshape that represents a group of objects).
The signer first named the noun book. Then, she describes the size of a book using a descriptive classifier along with the mouth morpheme "mm". That is, a number of the pages of a book is average.
Notice the head tilting. It also has its non-manual grammatical significance. It's part of the whole production.
Notice a different classifier from the previous one? Not only the mouth morpheme describes a size, but also the classifier describes a size! This time the signer mouths "cha". That is, the book is thick in pages.
The classifier handshape is about the same as the first one above. But, the mouth morpheme "oo" makes it all clear. It means that the length of pages of a book is short.
The mouthing describes a size of the crewcut. Of course, crewcut is short itself. But, with the additional grammatical information, it conveys as: "The crewcut is so short." Or, very short. Shorter than typically short.
The length of crewcut is moderate, not too short.
The shirt sleeve is neither too short nor long.
Think of the size of a mug of beer at Oktoberfest in Germany. It's huge!
The signer first signed the noun beer and then the classifier predicate (the verb phrase including a classifier).
Note that the handshape of this ASL adjective is usually referred to a human or a two-legged animal (e.g. Sasquatch), and sometimes four-legged animals.
Not only this adjective sign doesn't fit with a tall building, the mouth morpheme would be also incorrect. Use a different classifier and different mouthing ("aah"). Yes, every language has different rules for different contexts.
These mouth morphemes are some basics that ASL students learn to use. As signers improve over many years, they will learn to recognize more detailed meanings and learn when to use facial grammar (and classifiers) correctly.
For example, the morpheme "OO" doesn't always mean a small size. It has meanings other than a size. Other ASL signs come with this morpheme "OO". For example: so-cute, lightly windy, glittering, so-cold (temperature), short distance, etc.
Likewise, not all these mouth morphemes work with some other noun phrases. Some have different grammatical rules. For example, use the mouth movement "th" for a very thin paper but use "oo" for a thin pen.
Native signers are much more sensitive to nuances and subtle meanings in signing. For example, notice the pinkie of the classifier in the "short book" phrase above. It's slightly different from the "medium book" phrase above. It's not something one intently produces. It's all subconscious and native signers are sensitive to every subtle significance of the whole production.
Practice signing to improve as well as socializing in ASL community to acquire your signing skills naturally. Over the years of daily use, you will become more aware of the nuances than before.
Also see facial grammar describing sizes: CS, MM, AAH