Not overengineered or underengineered

Elegant is widely used in science and mathematics to describe a theory which exactly and economically describes the data. It is also used in engineering in the sense desired.

Reasonable, practical, sensible, level-headed? Well-designed? A solution might also be down-to-earth, functional, pragmatic or even commonsensical.

(these probably fit less, but you might find them useful anyway: suitable, appropriate, reasonably engineered, just right)

I've seen appropriate used as a marketing term for this idea, as in an appropriate solution or appropriate technology.

Personally I like right-sized but it might not be as understandable to an audience.