You can't do a marketing plan without getting many people involved. No matter what your size, get feedback from all parts of your company: finance, manufacturing, personnel, supply and so on--in addition to marketing itself. This is especially important because it will take all aspects of your company to make your marketing plan work. Your key people can provide realistic input on what's achievable and how your goals can be reached, and they can share any insights they have on any potential, as-yet-unrealized marketing opportunities, adding another dimension to your plan. If you're essentially a one-person management operation, you'll have to wear all your hats at one time--but at least the meetings will be short!
The plan should cover one year. For small companies, this is often the best way to think about marketing. Things change, people leave, markets evolve, customers come and go. Later on we suggest creating a section of your plan that addresses the medium-term future--two to four years down the road. But the bulk of your plan should focus on the coming year.