"That athlete has a nice body."
Translation:Εκείνος ο αθλητής έχει ωραίο σώμα.
This is a question that re-occurs. :)
It isn't "grammatically" wrong, but it IS weird and unnecessary, which makes it ultimately wrong.
It all depends on what you want to point to.
The strongest words here are ωραίο and σώμα (you'll stress σώμα, as if you're pointing to that great bod, mmm). Εκείνος ο αθλητής έχει ωραίο ΣΩΜΑ, μμμ! LOL
You can also say the same sentence and stress the word ωραίο.
But if you said ένα and stressed it, it would mean "one."