Attempting to translate:
Look at the size of them, will you?
I said:
Schau sich doch ihre Größe an, ja!
My German friends corrected me with:
Schau sich doch einer ihre Größe an, ja!
What "einer" is, and why it needs to be here, are my questions. The only other reference to this idiom I could find was in Reverso Dictionary under anschauen, where it said
(da) schau einer an! (inf) well I never! (inf)
which was not helpful. Please advise.
As an alternative to your friend's and @Janka's translation:
I guess that you actually meant "schau" as the imperative form of "schauen", like in the English version.
In that case, the sentence would just need to be:
Schau dir doch ihre Größe an, ja?
If you set a reflexive verb ("sich etwas anschauen") into the imperative form, the reflexive pronoun "sich" becomes "dir", like in the second person singular ("du schaust dir etwas an").
With "Sie", it would be:
Schauen Sie sich doch ihre Größe an, ja?
You sentence misses a subject that sich could reflect to. That's why you need einer. It's similar to man:
Schau sich einer das an!
Schau man sich das an!
The difference between those two is subtle. Sich einer is less definite and thus, almost sarcastic.
