To allow a start to be made let us assume the concept for the work is already in place. You know what you want to write, even if it is only a simple construct in your mind. The final version is still a long way off. At this stage allow your imagination considerable licence. All is possible.
Any journey starts with one small step.
The finished work can follow the accepted formats, or can offer some unusual perspectives. Full-length fiction normally assumes a work stretching beyond 60,000 words, so with about 400 words per typed page there will be over 120 pages in the pile of paper on your desk. Short stories are usually collected together to make a bound work that looks the same as a novel, something over 100 pages. Poetry books are often slimmer, a poem a page with just 30 works in a slim volume makes for a romantic tinge. Works of non-fiction, good and stolid, factually correct are of indeterminate length.