This is my third question for 101 Football Knowledge People. I'd like to know what size, speed differences (and other aspects) separate them. Once Downs joined the Buckeyes presumably moving Styles to LB got me wondering about this. 

As a side question, how narrow is the 40 times between safeties, linebackers and corners (with corners needing to be fastest I presume) ie. what is an elite speed for each?

BTW I learned a ton from you all on the question re: QB Arm Strength- thanks!