Every side of it
A 118 millimetre square that sits on a shelf and tells you what the air in the room is doing. Mains powered, e-ink faced, and readable from across a room without waking a screen.
It occupies about as much shelf as a paperback laid flat. The intake sits on the lower right edge, so pushing it back against a wall does not choke it.
No adapter, deliberately. Almost every household already has a spare USB-C charger, and shipping one more is the single largest part of this product's carbon footprint.
Tolerances are the figures we test production units against at 25 °C and 50% relative humidity, not the best case printed on a sensor datasheet.
Every unit is calibrated against a reference instrument before it leaves the line, and the certificate number is printed inside the battery-free base.
The carbon dioxide sensor then keeps itself honest with automatic baseline correction: it assumes that, over any seven-day window, the lowest reading it saw was fresh air at 420 ppm, and re-zeroes to that. If the device lives somewhere that never gets fresh air — a grow tent, a server room, a boat — turn that off in settings and calibrate manually instead.
£149 including VAT, from the official store or any authorised retailer. The directory says who ships to you and who handles warranty service in your country.