Where it goes in the ceiling matters more than its size. A small unit placed above the vanity does more than a large one over the bath, because the light lands where you stand. On the visit the basin goes onto the plan first, and the measuring runs outwards from it to the rafters.
Privacy is a common worry in a terrace with close neighbours. A roof window looks up, not across, so overlooking is rarely an issue, and where a higher window overlooks the slope, the glass can be frosted or obscured.
Steam and condensation need thought, too. A bathroom produces moisture, so we recommend a unit that opens, or a rooflight paired with an extractor, to keep the room dry. The aim is a bright room that stays fresh.
If the bathroom is the only room you are thinking about, say so, but mention the landing as well. In a house of this layout the stair and the bathroom often sit side by side, and one well-placed unit sometimes serves both. We look at that possibility before suggesting two openings, since every extra opening adds cost and complexity to the roof.
A mirror makes a useful tool at the survey. Stand at the basin and look up, and the point where you want the light to arrive becomes clear. We use that spot to draw the cone of daylight on the plan, and you can see on paper how it will fall.
A rooflight set over the basin works harder than a bigger one over the bath.