There's never been a successful receiver/tight end in the history of the league at that height.
http://www.grantland.com/story/_/id/...ty-success-nfl
"Perhaps the biggest thing that would hold James back from being an excellent football player for a long time would be the same thing that helps him excel as a mismatch in basketball: his height. James is listed at 6-foot-8 and 250 pounds, but there have been suggestions that he has grown since arriving in the league, to around 6-foot-9 and the 255- to 275-pound range. It doesn't really matter. In either case, while LeBron's height would make him a terror on jump balls1 in the end zone (as Bryant suggested), his height would also leave him susceptible to something he doesn't have to fend off in basketball:2 an endless stream of defenders diving at his knees and ankles. Health is a skill, and as a player, protecting yourself from injuries is part of the job. With James's size and strength, safeties would likely resort to diving at his ankles and knees to bring him down, increasing his risk of injury. It might not be physically possible for a player that size to protect himself from low hits downfield.
That is to say that there is virtually no precedent for a receiver as big as LeBron suiting up and playing effectively in the NFL. According to the height records from Pro-Football-Reference.com, there have been only six receivers since the merger who were 6-foot-8 or taller to have any sort of NFL career, and four of them (Brad Cottam, Greg Estandia, Joey Haynos, Zach Hilton, all 6-foot-8) lasted a maximum of three seasons. Morris Stroud, who played for the Chiefs, at 6-foot-10 is the tallest receiver in NFL history, and is responsible for the rule that prevents players from standing in front of the uprights to block field goals. He had 977 yards and seven touchdowns from 1969 to 1974, so while the league hadn't yet evolved into the passing competition it is today, Stroud wasn't exactly unstoppable in the red zone in the way that James would likely be. The only effective receiver near James's height was 6-foot-8 Harold Carmichael, who made four Pro Bowls for the Eagles from 1971 to 1983. James, though, outweighs him by at least 25 pounds.
What about all those basketball players who became top receivers, you ask? Well, they're nowhere near as big as James; many3 have the bodies of players who would suit up at shooting guard or as an undersize small forward in the NBA. Antonio Gates is 6-foot-4. Tony Gonzalez, the greatest tight end of all time, is 6-foot-5. Rickey Dudley, who, like Gonzalez, was a first-round pick of the Raiders and a college basketball player, was 6-foot-6. The closest comparison for James is likely Jimmy Graham, who is 6-foot-7 and 260 pounds. Graham is a very good receiver, but he also plays with Drew Brees, which helps eliminate some of his rawness. And that two inches (if we go with the 6-foot-9 estimate for James) might not sound like much, but if it weren't a big deal, why hasn't there been a single player taller4 than 6-foot-7 to emerge as a viable receiver since Ronald Reagan became president? It's rare to find an athlete sized like LeBron, but wouldn't one have come through and played professional football by now?"
He goes on to say James would actually be best suited as a defensive end if he were to play.