Solid oak dining tables in Hertfordshire.

The one county where the rooms repeat.

Hertfordshire is unusual, and it makes sizing easier here than almost anywhere. Letchworth was the world's first garden city, Welwyn Garden City followed in 1920, and Stevenage was designated the first post-war New Town in 1946. All three were built to plan, which means their houses repeat to a small number of standard types rather than varying street by street. If you live in one of them, we can usually tell you what will fit from the house type alone.

Those planned houses tend to share a trait: generous room proportions but modest original doorways, because they were designed before anyone owned much furniture. A 180cm × 90cm or 200cm × 90cm sits comfortably in most of them, and the doorway is the thing to measure rather than the room.

The rest of the county is the opposite. St Albans city centre and the Berkhamsted conservation area are full of Georgian and Victorian townhouses with deep, narrow rooms where 200cm × 90cm works and 100cm width does not. Harpenden, Radlett and Chorleywood run to large interwar detached houses with knocked-through kitchen-diners that comfortably take 240cm or 260cm at 100cm wide. And the flint-and-brick cottages around Tring and the Chilterns villages want 160cm or 180cm, with the door head measured first.