In order to have a good lenght for your URL, it is good not to have more than 3-5 words in your URL. According to Google?s Matt Cutts, if your URL have more than 5 words ?[Google] algorithms will typically weight those words less and not give you as much credit.?
With new Panda algorithm, introduced by Google at the beginning of 2011 in US, the importance of url as principal keywords container is increased. With URL you help google to distinguish unique content in your web site.
A study from Marketing Sherpa, 2008 Search Marketing Benchmark Guide, shows that short URL?s tend to be clicked twice than the longer ones on Google SERP (for an equal position rank).