My personal take is that it doesn’t really matter as long as you are giving enough information on your page. What’s important is the relevance. You don’t need to go on and one just because you have to write n number of words. But to be on a safer side I’d say you should have anywhere between 200-400 words and that too organised under proper headings. Try to have one main heading and at least one sub-heading (<h2></h2>) and a couple of links carrying your keywords as your anchor text and you are on a safe plain and after that your SEO depends on many other factors.
But why do some SEO experts insist on greater length of web page? The more you write, the more you can use your keywords, and that’s the basic idea. The keyword density should be anywhere between 2-5% of your total number of words and if you merely have say, 150 words, the 3% keywords in it will sound odd. When your text is longer you get more chance to use multiple sub-headings, bulleted lists and blockquotes and all these tags are good for SEO.