I should say knowing how long your story is going to be from the moment you've thought of it would have to be almost intuitive. It depends what it's about. Also a series with a continuing story, or even 'episodic continuing' as someone said before, requires a lot of plot structuring before you can even get started. J.K Rowling for example spent five years plotting out Harry Potter before writing the first novel.
I chose to make mine a trilogy because it's a 'quest for the macguffin' style story in which there are four identical macguffins, so each book could be about the pursuit of one at a time. It's taken me years to plot it out too, there's so much to think about in advance. There were always going to be three, but I've another project that'll definitely be a one off, and another that will be better as a collection of shorts![]()
In that case, I am a massive failure at the intuition thing.
'Labyrinth' was originally intended to be 10-15 K. First draft hit 44K. Went back, cut a lot of stuff, added things to fix characters and plot, and...62K. Then, I decided that some of the cut stuff was too good to leave out, so 'Labyrinth' became 'Labyrinth: Journal' and I wrote the sequel, 'Labyrinth: Seed.'
Then again, I did do a bit better with 'Empire.' That was intended to be a series of short, 60-70K novels exploring the changing scene in the Imperial Heartland. Thus far the first three books in that series (six or seven total) are all right around 60K, and once I finish the rewrite, book 4 will probably be about 70K. However, while each book tells a story, that story is part of a greater whole.