How big does a uterine fibroid have to be to cause severe back pain? severe back pain with weakness in left leg

Any: But usually those >4cm likely to be symptomatic. Any fibroid that is dying (necrosis) can cause severe pains. Doctors always should consider other causes, though, because the more common symptom of a fibroid is bleeding--not pain.

Not usual: A fibroid should not be causing weakness along with back pain. This sounds more like a disk and with weakness you need to talk with a physician sooner than later. Fibroids can cause pain in the back but NOT weakness.