**Core Concept**
Pituitary adenomas are classified based on their size, which is a crucial factor in determining their clinical behavior and management. The size classification helps differentiate between non-functioning and functioning adenomas, as well as to predict the likelihood of invasion and recurrence.

**Why the Correct Answer is Right**
Macroadenomas are defined as pituitary adenomas with a diameter of 10 mm or greater. This size threshold is based on the fact that larger adenomas are more likely to cause mass effects, such as visual field defects and headaches, due to their increased size and potential for invasion into surrounding structures. The pituitary gland is surrounded by the sella turcica, and macroadenomas can grow beyond this bony confines, leading to potential complications.

**Why Each Wrong Option is Incorrect**
**Option A:** Less than 5 mm - This size is typically considered microadenoma, which is a smaller pituitary adenoma with a diameter of less than 10 mm.
**Option B:** Greater than 20 mm - While larger adenomas are more likely to cause mass effects, the size threshold for macroadenoma is specifically 10 mm or greater.
**Option C:** Less than 5 mm in height - The size classification of pituitary adenomas is based on diameter, not height.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that the size of pituitary adenomas is a critical factor in determining their clinical behavior and management. Macroadenomas require more aggressive treatment and follow-up compared to microadenomas.

**Correct Answer:** D. 10 mm or greater.