Learn Swedish: Vi såg en val med en längd på 15 meter. - We saw a whale with a length of 15 meters.

When specifying an exact measurement, age, size, or price right after a noun, Swedish uses the preposition på. While English uses "of" (a length of 15 meters), Swedish frames it more like "a length at 15 meters" (en längd på 15 meter).

Why doesn't the word meter have a plural ending here?

In Swedish, metric units of measurement that end in a consonant, like meter, liter, and mil, usually do not change form in the indefinite plural. So it is en meter, but also 15 meter.

I've seen the word val used to mean "choice" or "election". How can I tell the difference?

Context usually makes it obvious, but they also have different grammatical genders! The animal (whale) is an en-word (en val, valen). The concept of a choice or election is an ett-word (ett val, valet).

Could I just say en 15 meter lång val instead of using this long prepositional phrase?

Yes, absolutely! Saying en 15 meter lång val (a 15-meter-long whale) is often more common in everyday speech. The phrasing med en längd på (with a length of) sounds a bit more formal, descriptive, or scientific, much like it does in English.