General small smiles:
* Smirk (can be slightly mischievous or smug)
* Grin (usually a broader, wider smile)
* Twitch (a very subtle, almost involuntary smile)
* Flicker (a brief, fleeting smile)
* Glimpse (a fleeting or partial smile)
* Hint (a subtle suggestion of a smile)
* Sugary smile (a sweet, but possibly insincere, smile)
More specific small smiles:
* Playful smile (a lighthearted, teasing smile)
* Shy smile (a hesitant, bashful smile)
* Wistful smile (a thoughtful, melancholic smile)
* Sad smile (a smile that masks sadness)
* Self-conscious smile (a nervous or awkward smile)
The best synonym depends on the context of your writing.