"Varför har hon så långa armar?"

Translation: Why does she have such long arms?

[sundoll]

Why??

[Lundgren8]

I guess we’ll never know.

[HejHenn]

Maybe she has Marfan syndrome

[Cris_Grey]

Why is “why does she have so long arms?” wrong?

[devalanteriel]

That's definitely correct. I'm adding that now.

[Steph.Ro]

It doesn't sound quite right, devalanteriel. so in this adjectival phrase doesn't work.

You can say things like Why does she have so much hair/so many freckles?

And of course long can be used in a predicative manner, as in Why are her arms so long?

so long as a set phrase meaning see you soon works, too.

But in the sentence at hand so long arms simply doesn't sound right.

[JeanLibera1]

Is "så" an abbreviation for "sådan"? Could "sådan" be used instead, and would it change the meaning of the question?

[devalanteriel]

No, så is a word of its own. English can use "such" in basically the same sense as "so" here, but Swedish sådana doesn't quite work as a substitute. You'd be understood but people would think it sounds quaint, or off.