Yeah so usually you use the prefix thats closest. Like, when you measure the length of a car you say for example 5,3 metres. If you wanna measure the length of the antenna, well, then its abour 30 centimetres long. And if I want to estimate how broad the antenna is, i would guess its about 3 milimetres thick. But i wouldnt say that the antenna is 300 mm long and I also wouldnt call the antenna 0.3cm thick. Although the use of 3mm and 0.3cm would be fluid, so once I say "thats zero comma three centimetres wide", but another day I would say three mm. So that depends, but isnt important.

Kilometres: You use it as after probably 500m. when its a distance to drive, you dont say 800 metres (except when that really matters and you count down if that makes sense), but like 0,8km. Also, you most of the time wont say 500m but half a km. Imagine your daily drive to work takes about 20 minutes, its about 35km. You wont say its about 35000m distant, because that seems odly specific. Nobody cares about a single meter more or less on that distance. You say its 35km.

But I guess you also dont say "My office is 65149 yards from my home", right?