Marcus Aurelius wrote:......the 54 would probably give you a ‘twitchier’ ride, in comparison to the more relaxed 56. …...
Why is that then? On many "road" bikes, manufacturers use a steeper head angle on the larger sizes in order to get a longer reach for the same wheelbase. Unless they also use a shorter offset fork, steeper head angle gives less trail, ie faster steering.
The same rider would be expected to need a shorter stem on the larger bike, and its often said that shorter stems give twitchier handling; it may be the case that less weight on the front wheel gives a lighter feel to the steering.
Riding no hands, a short trail bike is easy to (for example) slalom the cats' eyes, but may be difficult to keep a straight line due to random steering inputs from eg road surface. No hands on a long trail bike the bike wants to go straight on and requires exaggerated angles of lean to "steer" it. Stem length has no bearing on how a bike steers no hands.