As with most writing, clear and concise is the best for that web. The most important thing is to make sure you have the information on your website that your users want and that they can easily find it.
People 'skim' more online and have even shorter attention spans than they do reading on paper, so elements which help your audience to skim through your content to find what they are after will mean that they find your site useful and will keep them coming back rather than frustrate them and make them leave your site for good.
The length of your content depends on what you want your site to be used for. If yours is an information website, ensure a visitor can find answers to questions on your subject. If you are you offering a product or service, make sure your customers can easily find:
For a basic product or service website, generally a few paragraphs on each page is sufficient to get your message across without confusing your users. Start by simply stating your key points under each heading and you should be half way there.
General Writing Tips
Art Attack also offers copywriting services if you need further assistance, which can range from writing all your content through to 'massaging' yout text to make sure it is clear and works for the web.