We use it thousands of times during the day but have no real knowledge of what it means. What is a pound or a kilogram, a mile or a kilometer, second or an hour? They are all as most dimensions are, agreed-upon values that allow the sharing of ideas, understand them the ability to reproduce them while possibly improving upon them. This should be the first act upon your arrival, a standardization of measuring units. There should be only one for a given measurement. Take a rock and say shouting, “This is one <insert your name> unit of weight and uses no other”. Then take a pebble and say shouting: “If <insert your name> is too much use this smaller <insert your name> unit which we will call mini <insert your name>”.
Or take a step and shout: “this is one unit of distance which we will call one foot”.
If starting from the beginning why not take the opportunity to make a mark on scientific progress for millennia to come by naming measurement units after you? Like, besides no one will ever know that <your name> unit was in some distant future called by a generic name of the kilogram. It is a bit egomaniacal but at this point, you need an ego boost as you are in the 13th century.
Measurement, accurate measurement will is what enabled the Industrial Revolution in the first place. That and the idea of interchangeable parts is what made the modern world possible. To be able to create parts that fit together is the cornerstone of the industry, literally. The basis of accuracy is the marble stone block.