General

The term "small macromolecules" is an oxymoron, since something can't be both "small" and "macro" simultaneously. Still, we use these terms to talk about the sizes of molecules, from the very small (like dihydrogen) to the very large (like hemoglobin).  So you can have a small protein that would still be huge compared to hydrogen, for example.

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Arbitrary size of oligomers

Larger entities with more than 50 units are big enough to be "polymers" when the units are synthetic organic compounds, and "proteins" when the units are amino acids. Some protein chemists will insist proteins can be as small as 10 amino acids linked together, while some polymer chemists might insist a true polymer requires thousands if not millions of monomer units linked together. There is little dispute, however, that DNA and RNA are oligomers (sometimes referred to as "oligos") in this size range, where the monomer units are now nucleic acids.

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Proteins and peptides

A peptide is just a short protein, so a peptide is an oligomer with amino acid monomers. Protein (and peptide) chemists formed their own club initially and possibly the more generic term oligomer had not yet been invented. But as with oligomers, there is no strict definition for a peptide, and again, we will use the figure of >50 means protein, <50 means peptide, but it is also true a strong case can be made that certain peptides only 10 amino acids long (10mers) are actually proteins (see Satoh, et al. FEBS Lett. 580:3422 (2006))

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Fluorescence

This is the interesting phenomenon of light emission from a molecule, known as a fluorophore, where first the fluorophore is energized with incoming light, and then the fluorophore "glows" as it emits light of a color that is characteristic of the fluorophore. The emitted light is always of lower energy (longer wavelength) than the light used for excitation (the energizing light). Not many molecules are fluorophores: Once most molecules are energized, the energy that came in as light gets dissipated as heat.

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Analysis

An early goal was to make new GFP molecules that can be mixed with an external "analyte" (the material we want to analyze) and change its properties in proportion to the amount of analyte present. This way, the new GFP molecule can be used to tell us if an analyte is present, and importantly, how much is present. Analytes can be anything we want to focus on, like metals, drugs, pollutants, explosives, or pesticides. While this is still a possibility, recent results have pointed in the direction of peptide-binding-peptides and peptides capable of binding small molecules.