Can get very large! Fibroids can vary as to how many form and how big they can grow. These factors are dependent mostly on genetic factors. I have seen them form as large as a basketball in women in their 20s, or only be an inch or so by their 40s. Depending on the size and location, they may not cause any symptoms, or can cause heavier bleeding, pelvic pain, feelings of pressure, or limited bladder capacity.
There isn't a limit. Fibroids can get to be as large as basketballs or bigger, but usually they start to cause symptoms by this time, and removal may be recommended. Usually the symptoms are changes in bowel or bladder habits, irregular menstrual bleeding, pressure in the pelvis or abdomen, and rarely; pain.
Varies. Size of fibroids varies from smaller than a bb up to larger than a volleyball (a fibroid can be much larger than an average uterus!) common symptoms are heavier/irregular bleeding, and pelvic pain/pressure, --depends on size and location; see your gyn for an us to evaluate your fibroids and to discuss treatment options.