An organic fluorophore molecule (e.g. fluorescein) or a chelated lanthanide (terbium, europium) ion is quite small. The smallest organic fluorophore I can think of consists of a substituted benzene ring, such as the N-methylanthraniloyl group. Reduced nicotinamide is also fluorescent. Such small molecules are excited in the ultraviolet.

You should define the technique which you are interested in. Fluorescent, electronic transition is in general possible (Franck–Condon principle and Kasha's rule) and even used in laser induced breakdown spectroscopy (--> H2).

Quantum dots range in sizes from ~1 nm-10 nm.