Some believe they should be short, some believe they should be comprehensive. As an online merchant, your main goal regarding the product pages on your storefront should be to find out what your ideal customer wants and how best to reach them.
Review Customer Scenarios
Before developing your product pages, the first thing you should do is review the ways that your customers purchase products. Do they perform heavy online research before making a purchase? Do they go to an offline store to see the product, then go online to get a better price? Conversely, maybe your product is exclusive to your company (and therefore requires more detailed information). These scenarios and others should be taken into account when developing your product pages. So how do you develop a page that reaches various customer groups?
Web Page Format is Key
The format is one of the most important aspects of the product web page. The format should be consistent with both your product type and price point. After all, if someone is spending $1000 on a specific product should the page format and provided content be the same as a $2 product?
Compare the product page formats of Amazon.com to Buy.com. They are completely different. You will need to decide on how to structure (or present) the content, what content should be placed on the page, where to place the buy button (add to wish list etc...), how many images. and if you want to up-sell products on this page. The reason Buy.com product pages look so different than Amazon.com product pages is because Buy.com customers prefer it and have reacted kindly to it in tests. I recommend doing the same for your site. Create 3 - 10 product page variations and test them to see how your customers react to each. Use different formats, use various versions of the buy button, split up content offerings using popups, Javascript tabs, or a long page design, etc. This is really the best way you can find out what the optimal product page for you site truly is.
In the Details
The content should provide those who are performing research with enough material to allow them to make a purchase decision. Yet it should also not make the buyer question if they are purchasing the wrong product or model. Keep in mind that some people are easily convinced! Thus if you provide them with just enough information to confirm that it is indeed the correct product, they will purchase it. However, if you start sparking doubt in their mind then they may just leave the site altogether or the purchase may drag on for weeks.
Related Links
For more information, see these articles on creating a product page that sells:
http://www.marketingexperiments.com/improving-website-conversion/long-copy-short-copy.html
http://www.ecommerce-guide.com/solutions/building/article.php/3579196