In writing, sometimes a general term is enough. Other times, you will want to get more specific. The dictates of the story will usually determine it, and -- mark this -- it is better to be general and get it right than specific and get it wrong. This is an ongoing discussion here. Better to say, "She shot him with a gun she pulled from her purse." than "She shot him with a Taurus .38 Special automatic pistol she pulled from her crocodile Hermes bag."
"Gun" is boring, but it isn't wrong. No such thing as a Taurus pistol in that caliber.
I'm guessing about the handbag. Might have been a knock-off.
In journalism class, they warned us to beware of unqualified general conditions. "It was a modern building." Or, "He was tall." Or "It was cold."
What does "modern" mean? This month? This year? This century?
"Tall" compared to whom? Tom Thumb? Manute Bol? Joey the Giraffe?
How cold is cold? Water turns solid at (in F.) 32°. Nitrogen is a liquid at -196°. Helium is still liquid at -269° and might not get solid above Absolute Zero (−459.67°).
Below Absolute Zero theoretically doesn't exist. That's the basement floor.
Colder than a Bose-Einstein condensate? Or colder than a witch's tit? Or a well-digger's belt buckle? These latter are not precise measures, but they convey a certain ... tone. Cold enough to freeze the balls off a brass monkey? Colder than my ex-wife with a headache ... ?
Black as a crow's wing. Is this blacker than a pedophile's sin?
Gets to be more fun once you start waxing metaphorical.
Comparisons are a good way to give people a memorable visual. If I say "He had a bruise on the back of his hand 47 mm in diameter." that might not mean an awful lot to the non-metric crowd. If I say, "He had a bruise the size of a Liberty Head silver dollar on the back of his hand." and you know what that coin is, it's easier to visualize.
"Big around as a tennis ball," probably you have an idea how big that is. "Big around as a spliggle ball?" Good luck with that one.
One of the drawbacks in science fiction and fantasy comes from such comparisons. "Big as a pack of cigarettes." isn't one you can use in your fantasy novel set in Cimmeria. (Though it might be amusing: "Conan took a pack of Luckies from his direwolf pelt and offered one to Thunga. 'Coffin nail?' Conan asked cancerously.")
How far can Ooma chuck that spear in a world that doesn't use feet, meters, rods, or other Terran measures? How do you let a reader know? "Armspans" might work; even though that depends on how tall somebody is, you can get a general idea if Ooma is an adult and you've made him more or less human-sized.
"Paces" could do it. "Boot-lengths?" "Forearms?" That's where "foot" started, and it was a while before it was standardized to the current international length.
Have a look:
- 1 Amsterdam foot (voet) = 0.2831 m.
- 1 Danish foot (after 1835) = 0.31385 m.
- 1 French foot (pied du roi) = 12 pouces = 0.32484 m.
- 1 Norwegian foot (after 1824) = 0.31375 m.
- 1 Portuguese foot = 0.3285 m.
- 1 Rotterdam foot = 0.296 m.
- 1 Russian foot (English foot borrowed by Peter Ι) = 12 inches = 1/7 Russian sazhens =0.3048 m.
- 1 Spanish foot (till 1752) (Pie (foot) de Ribera/de Rey) = 12 Pulgadas = 0.287342 m.
- 1 Spanish foot (1752 to 1765) (Pie (foot) de Burgos/Castellano) = 0.278635 m.
- 1 Spanish foot (after 1765) (Pie (foot) de Rey) = 12 Pulgadas = 0.32483 m.
- 1 Swedish foot (fot) = 12 inches (tum) = 0.2969 m.
- 1 Venetian foot = 0.34773
How much draw does that bow have? Equal to the weight of a prepube girl half a span tall? Assuming the girl is not anorexic, nor overweight, you can get an idea that the bow isn't something a strong archer would have any trouble pulling, because the girl is almost certainly going to be at least two feet tall, and less than four feet. But if the bow has the draw equal to the weight of a larger than average man, now you are talking something Ulysses might string and shoot. (Yes, you have to define "average," and that could be tricky, but I'm sure you can come up with something relative, because that's all that really matters. A man who is stronger than any man in the village by half again? Doesn't matter how much weight he can push if he's wrasslin' with somebody in the village -- he'll be stronger than they are.)
These are things you need to work out, if you are going to sky off. Of course, you could use inches-feet-yards-miles and simply don't address it, and that's legit. You are, after all, probably writing in a terran language for an audience that presumably reads it, and there are some conventions allowed. If you make up a real alien language and write in it, your audience is apt to be limited, Klingon notwithstanding.
Just a few things about which to think as you cast your story and start the action ...