On Sat, 29 Nov 2014, Mark Szepieniec wrote:

So I guess my problem is resolved, but I'd still like to understand better
why my earlier solution of doing

\setupfloat[figure][width=2cm]

didn't set a default for later instances of \placefigure. I feel like I
might be missing some aspect of the ConTeXt philosophy, or I'm misreading
the manual, or it's just a bug...?

\setupfloat[figure] sets the values of \startplacefigure (and \placefigure), and not for \externalfigure. The width value for startplacefigure is not used anywhere.

The default value of \setupfloats[width=...] is used when the float box is empty. So, the only scenario where the value of \setupfloats[width=...] will matter is the following:

\starttext
\setupfloats[width=2cm]
\placefloat{}{}

\setupfloats[width=10cm]
\placefloat{}{}
\stoptext

Aditya