i'm almost certain this has been discussed in the past, but I don't
remember what had been decided.

We don't "decide" anything here, we just discuss.

Do we say {Duj 'ar wIghaj}, or {Duj 'ar DIghaj} ?

I'd tend towards the latter, since what we're actually asking is, "how
many ships do we have ?".

But I'd like to know, if there's a definite answer with regards to
this matter, or it's just another "use the force" problem.

'ar hasn't been used much. It appears in nIn 'ar wIghaj How much fuel do we have left? in several sources. In other places it's used the prefix doesn't distinguish singular or plural.

nIn 'ar wIghaj isn't terribly helpful, since nIn fuel is clearly a mass noun.

So unless there's an example I can't think of, we don't have anything conclusive. I tend toward Duj 'ar wIghaj, because the noun that 'ar is attached to is not allowed a plural suffix, so why not avoid a plural prefix as well?

-- 
SuStel
http://trimboli.name