What makes these smileys small
Most tiny smileys are built from modifier letters, the small raised letters that linguists use in phonetic transcription. ᵔ is MODIFIER LETTER SMALL TOP HALF O, ᵕ is its bottom half, and ᴗ and ᴥ are small Latin letters. Put them together, as in ᵔᴗᵔ or ᵔᴥᵔ, and you get a face about as tall as a lowercase letter.
Others use combining marks. ◡̈ is a lower half circle ◡ with a combining diaeresis on top, so two code points make one tiny face. The dots can sit slightly off-center in some fonts, which is part of the charm.
Single-character smileys
If you want exactly one character, use ☺︎ or ☻ (the classic smiling faces), ㋡ (circled katakana tu) or ㋛ (circled katakana shi). The APL symbols ⍤ and ⍨ also look like little faces, with two dots over a small circle or a tilde.
Some phones turn ☺︎ into a full-size emoji. ㋡ and the letter-based faces above have no emoji version, so they always stay small and match the color of your text. For the bigger classics and emoji, see the smiley face page.
Where tiny smileys work best
Small smileys fit where space is short: display names, Discord statuses, game chat, captions and the end of a text message. They look friendlier than a period and quieter than an emoji, for example "see you soon ᵔᴗᵔ". In work emails they can still read as casual, so save them for friends, chats and profiles.
You can build your own with the small text generator, which turns letters into superscript and small caps, or browse superscript symbols and kaomoji for more parts.