[Caprimaniac]

...."as the HB and the flywheel need to work as a pair, so need to be sized to complement each other"

We've agreed that both need the same imbalance or lack off such, so that's a sealed topic.

Regarding to total weight or physical size (diameter) I would say, it depends.
A lightweight flywheel/ clutch and a very heavy harmonic balancer or vice/versa- might not work out very well. I've never looked into it. It's interesting topic, though....

For instance; Should you put flywheel + clutch assembly on the scales and find a HB with approx same weight?

As I understand it, only the HB will absorb the harmonics/ resonances. (In addition to the rotating mass itself, down to the molecular scale...)

As the both serves as rotating mass at each end of a rotating stick, I suppose you are right that this also might take up some, in itself.

The imbalance in the rotating front and rears are there to minimize harmonics, so that's another story.