Can a single monomer be a macromolecule? If so, when?
I can't think of an example where a biological monomer would be a macromolecule.

Definitions of macromolecule vary, usually by molecular weight or number of monomers (repeat units). 

Personally, I'd go with ~1000 Dalton for a minimum, but the original definition of 1000 atoms is a good start too.

In any case, no biological monomer, including glucose will function the same as a macromolecule.

Consider starch - a macromolecule of sugars. It doesn't dissolve as quickly as simple sugar and has different physical properties.

Update

To clarify my comments.. Macromolecules or polymers are made up of monomers the way words are made up of letters. So no, a glucose molecule isn't really the same as a macromolecule, just like "R" is not a word.

Yes, macromolecules can be used to make larger assemblies like microtubules, filaments, etc., much the same way that words can form sentences and paragraphs.

In the polymer literature there's even the concept of a "macromonomer" referring to a monomer that is already large in size.

In my opinion though, the basic constituents (monomers) are still amino acids, nucleic acids, sugars, etc. These are not macromolecules.