**Core Concept**
A giant congenital melanocytic nevus is a rare type of large melanocytic lesion present at birth, typically covering a significant portion of the body surface. These nevi are often associated with an increased risk of developing melanoma and other complications.

**Why the Correct Answer is Right**
Giant congenital melanocytic nevi are usually larger than 20 cm in diameter, often covering a significant portion of the body surface, including the trunk, back, or limbs. This size criterion distinguishes them from smaller congenital melanocytic nevi. The large size of these nevi is due to the proliferation of melanocytes during fetal development.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect as giant congenital melanocytic nevi are not typically smaller than 10 cm in diameter.
**Option B:** This option is incorrect as giant congenital melanocytic nevi are not usually confined to a single limb.
**Option C:** This option is incorrect as giant congenital melanocytic nevi are not typically smaller than 5 cm in diameter.

**Clinical Pearl / High-Yield Fact**
Giant congenital melanocytic nevi are often associated with an increased risk of developing melanoma, particularly if they are larger than 20 cm in diameter. Regular surveillance and monitoring are essential for early detection and management of these high-risk lesions.

**Correct Answer: D. Larger than 20 cm.**