mobile.engadget.com is a 6 character word. It is a good domain length and you should make sure that your domain name is easy for people to type and remember.
Top Level Domain
mobile.engadget.com is still good but not best as same as .com. This is designed to target a specific country or audience. Remember that you can lose some web traffic to .com version of the domain because people tend to type .com more than any TLD.
a dictionary word. Excellent choice! Generally, people will remember dictionary words better than without.