|B5 notebooks by Green World
I used to use Clairefontaine Age Bag A4 notebooks and I do still have a small stock of them to use up, but over the last year or so, I've shifted to B5 as a preferred size. It's halfway (roughly) between A5 and A4 (179mm x 252mm or ~7x10"), so big enough to get a decent space to write in but not so large that it takes up too much real estate on my desk.
The only problem is that B5 isn't a very common size to buy in the UK.
So far with the current book, I've used a Stalogy 016 notebook (used for general plot ideas and structure) and a Leuchtturm dot grid (used for scene planning and 'thinking' - noodling about, trying to work out things). The Leuchtturm is now full and I've moved into a Green World B5 notebook for notes.
I bought the four-pack of these from Amazon last year. I just found them again on Amazon and they're currently much more than I paid for them. Maybe they're one of those products where the price varies quite a bit. Considering that they weren't terribly expensive (at least, not when I bought them) they have some really nice features, though fountain pen friendliness isn't high on that list!
But the feature I really like, is that on the top and bottom lines are small marks, 10.7 mm apart (I'm not sure why they used that spacing, but that's what it is!), with 15 marks across the width of the line. If you're not looking for them, they're almost invisible, but if you're wanting to draw a table or split the page vertically, they are incredibly useful.
There are 76 sides (38 sheets) and the binding is sewn so the book will lie pretty flat. 72 of those pages are lined - the other two are the front and back pages and are plain. Paper colour is off-white/ivory but not too yellow.
Because they're fairly slim notebooks, I can fit a few of them in my B5 writing Traveller's Notebook (converted from an old Filofax Deskfax - see here for more on that).
I originally bought these notebooks for when I'm at that stage between 'first ideas' (which tend to go in a general ideas notebook) and 'yes this is definitely going to be a book' (which gets a notebook with more pages in, usually). One of the others from the four-pack is currently housing the notes for the next book! I hadn't expected to need more than the two notebooks for the current book, but I've ended up making more notes and drawing more things in the Leuchtturm than normal.
They are really sweet books, with a decent number of pages and a good page layout. If only they were fountain pen friendly... but perhaps you can't have everything.