I think a 19ft trailer would be fine. Just watch the first few feet off the 89a into the entrance - there's a few feet dropoff that may give you trouble with a larger rig. If you stay to the SW of the entrance (furthest from Lee's Ferry) off 89a, the entrance off the road is much more level.
If you go left at the fork at the research station, there are a number of large spots on the edge of the canyon that I believe you could turn the trailer around. "Going left" does have some rocky terrain, so go slow and take your time. No true pull through spots, but look more for clearings large enough to turn around. Enjoy.