My own WIP is set on Earth, but it's a fantastic enough scenario that I don't want to always use conventional units of measure, such as yards and feet. I decided to explore the alternatives and came up with phrases like "stride length," "hand span," and "and arrow shot away."
Looking at historic terms for measurement can be helpful, whether you use them as is, or modify them for your fantasy world. This article of old-timey measurement may be of help. Did you know the old Spanish unit, vara, appeared on many U.S. land deeds? The exact length varied from place to place, but in California it was set at a little over 33 inches.
If you invent units or use ones wholly unfamiliar to readers, you may want to give them a clue as to what they mean or mentioning measurements will serve no value.
To remain vague may work to your advantage, as in, "The Orange Duke's castle was a day's ride away." Use words such as- dawn, dusk, twilight, sunset. Better yet, describe- "Our shadows were lengthening as rode across the desert." Of size- "about as big as a squirrel," or "an acorn size __ ."
For non-earth stories: Do you have a 24 hour rotation to your planet? If there are two suns, are they always in the sky at the same time? (Research binary star systems.) Does each sun have a name? Do you have four distinct seasons. More? Fewer? How will you give the reader a feel for that? "It was the season of phacelia, when the purple blooms carpet the landscape, a welcome sight after fifteen months of snow. But I shouldn't complain. Winter was short that year." (Dumb, tossed-together example, but you get the point.)
Remember, as a science fiction writer, you must know more about your planet's astronomy, geology, etc. than the reader. He/she doesn't need or want science and history lessons. They want a story and to get a feel for your world.
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