Arm's length is correct, meaning twice the length of one arm. So here "two arm's lengths" is what you need to say, even if the double plural may sound a little strange at first.

Thank you very much!

FWIW, das geht so auch im Deutschen "auf zwei Armlängen Abstand" ...

two arms' length

hätte ich hier erwartet.

It's kind of an odd thing to say, as when you are talking about being an arm's length away, you're imagining reaching out your arm and seeing how far it reaches. If you reach out both your arms, it's the same distance.

If you imagine a person reaching out both arms on either side, their body is still just one arm's length away from the things on either side. You'd have to imagine two disembodied arms :)

No baby elephant-distance outside of Austria? :-(

Though I suppose it could be two people each reaching out their arms.

I've seen these distances described in various ways, though, and they often sound quite odd :)

The UK gov. had a lot of stuff about 2m being the length of a double bed, for instance. Why it had to be a double bed, and not single or king-sized, wasn't explained!

I also saw "the length of a sperm whale penis", which allegedly is 2 m long ...

Beds used to be only 75 inches long (= 1,90 m) in the UK, I remember. Maybe they're longer now.

Bed lengths: I don't know if it's still the same now, but years ago a single bed used to be 6ft 3in (1.9 m) and a double bed 6ft 6in (1.98 m) long.

There was also a discussion here a while back on the plural of "ram's skull".