Is there a "design" or "practical soldering" rule-of-thumb for the
sizes of connecting wire?
I think it’s more a judgement thing, which you get a feel for by
trial and error. Good design is all about visual balance of both
components and space between them, ie positive and negative space.
Maybe it was because the piece was fairly large (about 65mm X
I think you’ve answered you own question really. If there was a rule
of thumb, then what would look good in a large piece, would look too
thick and “clunky” in a smaller piece, and vice versa. So for
smaller pieces, you’d probably be best to use finer gauges of wire,
and in larger pieces, heavier gauges.
Personally, I would do a drawing to the size you want to make the
piece, and estimate what gauge you think would work. Then lay the
pieces out and see if it works for you, before soldering the
elements together. Hopefully there won’t be any waste, as if you get
it wrong the first time and decided to change gauge, the pieces
you’ve cut can be used for another project.