Free tool
What size, and why
The two questions a shop would ask you are shoe size and where you plan to ride. This asks those, plus two more, and tells you what to look for.
Setup finder
Four questions
Four questions, then a straight answer about what width, what bar height, and which of the boards and scooters we have written up actually fit.
The short version, if you would rather just read it
Deck width follows foot size, not height. An EU 37 foot wants something around 7.5 inches, an EU 40 around 7.75 to 8.0, an EU 43 around 8.0 to 8.25, and anything bigger goes wider again. Your toes and heel should reach the edges of the deck without much hanging over.
After that, narrower flips faster and wider feels steadier. Street skaters take the narrow end of their range and people who ride ramps and bowls take the wide end. Nobody has ever been held back by being a quarter of an inch out.
For a stunt scooter the equivalent number is bar height, and it should land between the waist and the bottom of the ribs. Buying tall to leave growing room is the standard mistake and it makes tricks harder rather than easier.
For a small child on a first scooter, ignore the age on the box and find the weight limit. It is the number that decides how many years you get.