As it happens, the Steelers OLBers are notoriously under height for the position average.
Here's are some measurements for successful 3-4 OLBs (or who've played OLBs previously):
James Harrison 6-0 242
LaMarr Woodley 6-2 265
DeMarcus Ware 6-4 257
Tamba Hali 6-3 275
Cameron Wake 6-3 250
Clay Matthews 6-3 255
Calvin Pace 6-4 265
Shaun Phillips 6-3 250
Terrell Suggs 6-3 260
Kamerion Wimbley 6-4 255
Brian Orakpo 6-4 260
Bryan Thomas 6-4 265
Joey Porter 6-3 248
Shawne Merriman 6-4 262
Lawrence Taylor 6-3 241
The average of these players would be:
6-3 256
While Daryl's height at 6-2 wouldn't keep him from finding success, his weight at 230 is significantly less than the average.
Measureables aside, what makes for a good 3-4 rush backer is a combination of the ability to get to a fast speed quickly, and have the strength in his legs and core to deal with outside forces deliberately trying to force him off his path. He also needs to have enough upper body strength to shed punches or rip through other types of blocks.