I do crossword puzzles; I've been doing them for 30 years. These days the only ones I like are the Saturday puzzles from the Times, which are 1) hard and 2) not full of puns, a way of thinking I cannot comprehend, which makes it impossible for me to do British puzzles. I only read newspapers online these days, but I like to do crosswords on paper. So I do them out of books.
I got a new such book this Christmas and have been working my way through. I was just marveling at the strange way my brain knows instantly if a word or phrase I think of will fit the space I see. I can make this calculation without being able to tell you how many letters are in the word or how many spaces are in the row. I just know right away if a word or phrase will fit, and these instincts are never wrong. I recently did this with NAVALENGAGEMENT, so I can do it for 15 letters.
I have no idea how. Since I don't know the number, I can only guess that some automatic module in my mind takes my visual memory of how the word looks written out and lines it up with the blocks I see on the page.
I don't really feel that it is me doing this at all; it just happens, without my having anything to say about it.