Which size of Fibroid is dangerous, Uterine fibroids can vary in size from as small as a seed to as large as a melon, and they can develop as multiples. Assuming a single fibroid may be less than an inch, groups and clusters of fibroids can be much larger.Depending on the fibroid size classification and location, fibroids can affect various parts of your body, even expanding the uterus so that it impacts other organs or blocksthe fallopian tubes, which may result in difficulty conceiving.