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.
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.
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))
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.
Analysis
An early goal was to make a new GFP molecule that can be mixed with
an external "analyte" (the material we want to analyze) that changes
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 folding and
peptides capable of binding metals, but more for the analysis of the
protein, and less for the analysis of the metal.