Formal:
* Je suis assez petite/petit. (I am quite small.)
* Je suis de petite taille. (I am of small stature.)
Informal:
* Je suis plutôt petite/petit. (I am rather small.)
* Je suis petite/petit comme un/une... (I am small like a... [e.g., "comme un chat" (like a cat)] )
Adding a bit of emphasis:
* Je suis vraiment petite/petit. (I am really small.)
* Je suis très petite/petit. (I am very small.)
Remember:
* "Petite" is used for feminine subjects, "petit" for masculine subjects.
* You can also use "minuscule" or "naine" to emphasize smallness, but these are more informal and might sound a bit harsh.
Example:
* Je suis assez petite, mais je suis forte. (I am quite small, but I am strong.)