Is there a word or expression for a small crush on someone?

I'm not sure which scenario you want answered. But, you could say something along the lines of:

• My first sight of her made my heart flutter.
• My first sight of her set my heart aflutter.
• My first sight of her left me momentarily breathless.
• I had a thing for you when I first met you ...
• I had a little crush on you when I first met you ...

You can also exercise your imagination and use a little wordplay to convey similar sentiments. For example, hearts can take huge, little, and tiny leaps of joy, start beating loudly or faster, pulses can race, etc., etc.

How about "thrill"? American Heritage® Dictionary of the English Language, Fourth Edition, reads: " v. To cause to feel a sudden intense sensation; excite greatly. v. To give great pleasure to; delight. See Synonyms at enrapture. v. To cause to quiver, tremble, or vibrate. v. To feel a sudden quiver of excitement or emotion. v. To quiver, tremble, or vibrate. n. A quivering or trembling caused by sudden excitement or emotion. n. A source or cause of excitement or emotion."

I like to think of that momentary spark as someone being struck by Cupid's arrow.

The origin of the power of Cupid's arrow comes from Greek mythology, from the story of Apollo and Daphne: http://www.online-mythology.com/apollo_daphne/

Based on comments, it appears that sparked my fancy might be a good fit for this sudden warmth towards someone that doesn’t quite reach the mindless madness of a full crush.

Here are a few other somewhat related possibilities that come to mind:

• have a touch of ardor for
• an endearment
• a fancy
• feel disposed towards
• have a feeling for
• have some fondness for
• have a hankering for
• inclined towards
• have an itch for
• have a longing for
• be partial to
• puppy love
• hold in high regard
• take a shine to
• have a soft spot for
• have a strong fondness for
• tenderness
• have a thing for
• have a weakness for