Appropriate word for smile

Solution 1:

I think modest smile would work well:

Are you in love with that girl' she asked with a modest smile.

According to TFD it has the meanings:

1. Having or showing a moderate estimation of one's own talents, abilities, and value.
2. Having or proceeding from a disinclination to call attention to oneself; retiring or diffident. See Synonyms at shy1.
3. Observing conventional proprieties in speech, behavior, or dress.
4. Free from showiness or ostentation; unpretentious. See Synonyms at plain.
5. Moderate or limited in size, quantity, or range; not extreme: a modest price; a newspaper with a modest circulation.

By 5, it means "not large," which can mean small. "Not revealing much" may be captured by 4, particularly about herself as noted in 2.

Two other words to describe her smile might be coy or demure.

Solution 2:

Are you in love with that girl, she asked with a nervous smile on her face.

She is uncertain of the answer and is not sure of his reaction, hence nervous. I would have said hopeful or anticipatory but they imply that the verdict would be in her favor.