Littlest is a synonym for smallest. It begins with the letter L.
least
a or I The letter "a" as in... The man has ahat.
Qua is a word. It begins with the letter Q.
littlest
molecule
cell
The one-letter words I and a are pretty short.
DNA
Yes, 'worlds' is a real word.
The smallest compound word may be into.
Fa is a noun. It is the 4th note of the music scale. It begins with the letter f.