In the absence of clues, it is a judgment call.
Often, the problem statement or answer choices will tell you the desired units.
In general, units are available so that you can express dimensions in 4 or fewer digits with appropriate units. There are a few gaps, but the reason units are what they are is to make expressing everyday values easy to do.
SI units are easy to work with on this score, as the various available prefixes ensure that you can always express a quantity using a number between 1 and 1000. This is generally what you want to do (in the absence of direction otherwise).
For some things, there are customary units that are expected. Vehicle speed is unusally miles (or kilometers) per hour, for example. Land area is usually square feet, acres, or square miles in US units, or square meters, hectares, or square kilometers in SI units.
Generally, too, it is appreciated by your reader if the units you choose are ones they are familiar with. In most applications, you'd choose feet or yards or miles instead of furlongs, for example (unless your audience is familiar with racetrack distances).
