We determined that Character length on meta description code of 100magento.com web site is 194 character. Because search engines read average till 150 characters, If you put in many character , It will cause to be your sentence half. By the way, ıt can be reason to visitors who want to visit your web site from search engines, to click your web site. If you want to get too many visitors to your web site, you should launch your web site wery well in search engines result pages. You have to write sentence about your web site which sum up your content to your web page's meta description code. So you shouldn't use keywords in this section.