Width is the decision most people start with and the one that matters least. Depth is what a bathroom actually feels, because depth comes straight off the walkway. A 550mm vanity on one wall of a 1,600mm room leaves a little over a metre to stand in, and a door swing and an open drawer take their share of that. Settling depth first — 550mm, 450mm or 400mm — usually narrows the width to one or two sensible answers.
Where the basin sits along the counter decides whether the counter is usable. Centring it is the default and often wrong. On a vanity running into a return wall, a centred bowl splits the top into two strips, neither long enough to hold a folded towel. Pushing the bowl toward the wall end leaves one continuous run instead, and roughly 400mm of clear counter is where a surface starts being used rather than merely wiped. What limits the move is elbow room: a centreline closer than about 350mm to a return means knuckles against tile every wash.
Overhang is not a styling detail. Twenty to thirty millimetres past the face of the drawer fronts gives water a line to leave from, so it drips clear to the floor instead of running down the front and standing in the joint at the top edge of the panel, which is where a painted board front swells first. Beyond about 40mm the overhang becomes something to catch a hip on.
Height is quoted to the top of the counter, which is the wrong surface to measure. Hands meet the rim of the bowl. An undermount sits below the top so the two are near enough the same, while a semi-recessed or vessel basin adds 100 to 140mm and the cabinet has to come down to absorb it. Around 800mm suits most households and taller ones are easier nearer 900mm — but the figure is set from the rim downward.