Measure the arm of the chair, not the seat. A side table wants its top level with the arm or a little below it, so a glass can be put down and picked up without looking. Above the arm and you lift over an obstacle every time; well below it and you reach down into a gap.
Most armchair arms are 55cm to 65cm from the floor. Sofas with low, wide arms can be 45cm. A wingback with a high arm can be 70cm. One measurement with a tape decides which of the eleven tables below is right, and it takes less time than reading the rest of this guide.
The heights in the range
Six of the range are 60cm tall: the Camden Round, the Serene Rattan, the Abberley White, the Grafton Black, the Neatham and the Elmley Grey. That is the height that suits the majority of armchairs, and if you are buying without measuring, it is the safest guess.
Two sit lower. The Norton Black Square End Table is 48cm and the Hampton Ivory Shagreen Nest of Tables is 54cm, both of which suit a low-armed sofa or a deep lounge chair — and the Norton at 48cm doubles as a small coffee table in front of a two-seater.
Two sit higher. The Rutland Side Table is 60cm on a 60cm round top, and the Leckford Ribbed Black Oak Occasional Table is 76cm — tall enough to work beside a high-armed wing chair, and tall enough to stand alone against a wall as a small console.
Footprint: how much floor you are giving up
Width matters less than height but it decides where the table can go. The slim ones are the Grafton Black at 50 × 32cm and the Abberley White and Neatham at 40 × 40cm, all of which slot into the gap between a chair and a wall.
The larger ones want to be seen: the Rutland and Serene Rattan at 60cm across, the Leckford at 75cm, the Norton at 56cm square. A 75cm table between two armchairs is a shared surface rather than a personal one, which is a different piece of furniture doing a different job.
One table, or a nest
A nest of tables is the answer to a room that changes through the day. The Hampton Ivory Shagreen Nest of Tables is two tables at 54cm, which stack into one footprint and separate when there are people in the room. In a small sitting room that is more useful than a single larger table, and it is the one piece here that solves the problem of not knowing where the table needs to be.
The pieces in this guide

Norton Black Square End Table
£299

Hampton Ivory Shagreen Nest of Tables
£548

Abberley White End Table
£330

Neatham End Table
£357

Grafton Black End Table
£300

Elmley Grey End Table
£518

Camden Round Side Table
£91

Serene Rattan Side Table
£162

Serene One Drawer Side Table
£189

Rutland Side Table
£247

Leckford Ribbed Black Oak Occasional Table
£563