I think in that case it would be " Sunt mic" because it's singular? Not sure tho.
Depending on the speaker's gender, it would be "sunt mică" or "sunt mic".
My understanding is that this sentence is referencing that "They are little/small" It is indicated because of the adjective being used 'mici' which is the masculine plural form. "Sunt mic" would indicate that "I am little' because it is the masculine singular form.