Or, to translate (heh, math joke) that for the OP’s question: any larger than 16x16x6 is definitely wasted space, since that’s where the measurement polygon “maxes out”
However, @Wouter_Sikkema raises an interesting point – when you consider the hearthlings’ view distance, at 16x16 rooms they can’t see the decorations on the far wall if they’re right up against the opposite wall. So if you want to really min-max things, the best trick is probably to go smaller than 16x16, and “cushion the edges” of the room with path-blocking furniture/decorations that will stay below the hearthling’s eye level (e.g. vases, plush bunnies or braziers) so that hearthlings stay in the middle of the room and can never “squish” the polygon against a wall in a way that feels cramped.
Another trick with a particularly Evil Genius vibe would be to dig out the floor sections 2 blocks deep close to the walls, as an alternative way of keeping the hearthlings to the center of the room. It’s cheaper in the short term than packing that space with decorations… and hey, later you could fill those areas with water using a wet stone or just cover the gaps with carpeting – the hearthlings will still stay away from the walls, but you won’t have to look at the “comfort moats” hahah!