On average, a good business plan will be about 15 to 20 pages. Keep it concise but detailed. A good business plan will pay dividends both in the short and long terms. An investor and/or bank manager will see this and if the plan holds up well, then this will bring you the business that you seek. And from a personal point of view, a great business plan is something that you can return to time and again as a good reference point - something that you can draw upon in the future.
Another point of contention is how optimistic you should make your business plan. The problem with making over-optimistic predictions and plans is that these plans could well go wrong. Unforeseen circumstances can always put a spanner in the works. Therefore, it's wise to err on the side of caution when it comes to devising your business plan. Indeed, it's wise to predict conservatively and prudently when it comes to assessing the possibilities of future targets, sales and profits. A worst-case scenario will always prove to be less of a shock than one that raises expectations too high.