he's too small.

Diles is listed at 6' 240
Here are the measurements for other LB prospects:
Abbate, Jon 5-11 245 Wake Forest
Blades, H.B. 5-11 237 Pittsburgh
Davis, Buster 5-10 244 Florida State
Harris, David 6-2 239 Michigan
Siler, Brandon 6-2 238 Florida
Willis, Patrick 6-1 240 Mississippi
Alexander, Rufus 6-1 227 Oklahoma
Beason, Jon 6-0 236 Miami (Fla.)
Everett, Earl 6-2 234 Florida
Posluszny, Paul 6-1 237 Penn State
Timmons, Lawrence 6-3 232 Florida State
http://www.nfl.com/draft/profiles