I understand the answers provided by other members so far, and would not argue with any of them in various contexts. Since this is a one-person, home-based start-up (which I assume you are funding yourself), my suggestion is that you can eliminate a lot of the "standard" business plan content that is designed to explain your business to other people (lenders and investors). Yes, you'll need all that someday, but a lot of the relevant detail that describes your business and key operating procedures (stuff that others will need to see in the future) will be a work-in-progress anyway, and it will change/evolve before you reach that next stage.
What you
do really need is a good marketing plan (outlining exactly what you'll sell, how and where you'll get it, how much it will cost, who else is selling it, what competitive advantage you have over them, who will buy it, how much of it they will buy, how much they will pay, how they will know you're selling it, how they will pay for it, etc., etc.). It takes at least 4 to 7 pages to write a decent marketing plan for even a very simple business.
You also need a financial plan, especially a 12-month profit & loss projection and a detailed cash flow projection. A page or two (spreadsheets) should cover that.
Finally, you need to list and calculate your start-up expenses and capitalization. This is a good idea because people almost always underestimate these -- plus, you need to know your start-up costs for tax purposes. (You'll probably transfer some personal property to your business, and how you do that may have significant federal, state, and local tax consequences -- it's better to plan ahead and run the numbers now than discover you missed an opportunity or did something costly when you figure next year's taxes.) A page should cover it.
So overall, I'd think an 8 to 10 page plan would be a reasonable length for your start-up. I've gotten bank loans and credit lines with plans that that were less than 20 pages, and investment capital with a plan that was 46 pages -- so ultimately the plan needs to be however it long it needs to be to ensure your success.
Best wishes in your new endeavor!