It depends what your aim is and who your target readership is.
The advice that has stuck in my mind (gleaned from How to be a writer books) is that an average of 15 word sentences but to mix in short and long sentences.
After reading your post I did a google search to see what reason someone might advocate long sentences. The page I found basically said that short sentences set the reader up to expect a contrasting long sentence and the writer should use that long sentence to pack in a lot of action. I found that especially striking because I had just re-read Anabasis 1.5.8.
This section has 3 sentences. The first sentence is 18 words and sets the scene: A wagon has got stuck in the mud Kuros snaps that his highborn followers had better get the wagon free quickly.
Then comes a short 8 word sentence ἔνθα δὴ μέρος τι τῆς εὐταξίας ἦν θεάσασθαι. (“Then might be seen a specimen of their ready obedience”).
Then he hits the reader with a huge 61 word sentence in which he describes how the nobles round Kuros rush headlong into the mud with no regard for how their fine clothes will get dirty and the wagon is freed in no time. The very fact that this all happens in one sentence coveys the speed of action.
Of course it wasn’t Xenophon who decided where to insert the full stops but lets assume that the Byzantine scribes who did were able to guess where Xenophon would have put his full stops.
But Coderch is not claiming that Greek writers occasionally slipped in a long sentence now and then. When he says that the short sentences of the French original would not sound Greek if he used the same structure for the translation. He is saying the average sentence length in Ancient Greek is longer than French. Donovan also argues the same justifying his claim that this the result of Ancient Greek being a synthetic language: http://kart-hadasht.co.uk/anc/greeklang/greekcomp.php#sec2.
I calculate the average sentence length of the first book of Xenophon’s Anabasis as 24 words so on the face of it Coderch and Donovan are right.
But just because advice-to-writers books recommend a sentence average of 15 words doesn’t mean all English writers stick to it. See here for some examples:http://countwordsworth.com/blog/average-sentence-lengths-of-famous-novels-and-authors/
However, even Moby Dick, by Herman Melville at 21 is well short of Xenophon.
But who is Xenophon writing for? Those in Ancient Greece who could even read were a small section of the population. But underlying attitude of Xenophon’s Anabasis is starkly elitist. Greek leaders who while outwardly pretending to respect democratic forms manipulate the men into doing what needs to be done win his admiration. It is likely that Xenophon writing for a highly educated elite rather than the average literate citizen. Long sentences are arguably appropriate for such a readership.
But Antoine de Saint-Exupéry wrote for children. Hence he used much shorter sentences. We don’t (as far as I know) have any Ancient Greek Children’s books but to me it is obvious that if one turns up it would have sentences appropriate for children learning to read - that is to say with short sentences like those of Antoine de Saint-Exupéry rather than the long ones of Coderch’s translation.