“In a hole in the ground there lived a hobbit” - J. R. R. Tolkien
That is just how all good things start; with a small idea that grows. Same with anything creative.
Same for Game of Thrones by G.R.R. Martin… He’s stated that the whole story started from a vision of the “king’s party comes across a dead direwolf surrounded by her whimpering pups and the stag that pierced her chest” scene.
I’ve never seen my creative side explained so eloquently lol
“Excerpts from a book I’ll never write”
Oh look it’s me and my sexual fantasies
Short stories and flash fiction avoid this problem by having genre conventions that allow for focusing only on a specific scene. Ploughshares in its recent Winter edition has a fiction piece that’s three paragraphs long, which I thought was an appropriate length for the content.
I think it’s somewhat common for writers to start with writing shorter pieces. GRRM comes to mind.