Heptans are a unit of measurement in the world of Daventry.
Daventry was four hundred eighty-seven heptans square, and was said that Daventry is but a small spot surrounded by many larger nations. It is unclear if large it grew beyond that or if it remained approximately the same size.
Behind the scenes[]
A heptan is a term in some languages (german, polish, czech) that relates to certain molecules in organic chemistry. This has little to do with the King's Quest use of the word.
It's not clear what a heptan is in compared to metric or imperial measurements. Though there are some hints about Daventry's size in miles. The edge of Daventry to the east is said to be four miles a way from the castle (about a 'league') in Kingdom of Sorrow. See No Weevil suggests it only takes about a day or two to travel from north to south across the kingdom. At least in the section he was crossing over. In The Floating Castle the western edge of the kingdom is a few miles to the west, and the northern edge several miles to the north. In See No Weevil the southern edge is only a few miles to the south (with an exception that one road must head south between the Old Woods kingdom to the sea where Daventry keeps its poort.