Well, as much as I've complained about Tony Allen over the years and over the past few hours, I will say that the argument that he (or Giddens for that matter) can't guard LeBron is a fairly weak one. As well as Pierce and Posey played against him last year, LeBron is still LeBron, and when he's hot, no one's going to stop him. So to suggest that Tony Allen's height is going to do him in against him is probably not all that accurate because no one's really going to stop him.
It's thinking like that that lead teams to draft large stiff after large stiff in the '90s to try to "stop Shaq." All that lead to was an inordinate amount of large stiffs in the NBA because anyone who was big enough to bang with Shaq sure as heck wasn't quick enough to keep up with him, which made the point moot. Worse yet, most of those big stiffs were so offensively challenged that it actually made things worse, as it gave Shaq the freedom to help off his man on the defensive end. The worst aspect of all of that is you only have to face Shaq so many times a year, so teams were left with most of their games not even needing their big stiff, who did nothing but hurt them against other teams.
LeBron's kind of like the Shaq of small forwards, given his size and strength. While we can approach him like we have to stop him, we're probably better off just playing our game and trying to have the best possible players out there. If Tony Allen can harass him to the best of his ability defensively and make LeBron work on the defensive end, he might do OK.
Still, I'd be interested in a guy like Darius Miles. Not only does he have phenomenal length and size, he'd also is probably the only unrestricted free agent that has the clear cut talent to be our sixth man. He'd be a risk, but even if he's only mediocre he'd probably be a better option than most of what's left out there.