Final answer:
The lateral view is best for determining spleen size in a 99mTc-sulfur colloid liver/spleen image due to a clear view of its three-dimensional shape.
Step-by-step explanation:
The best view for determining the size of the spleen in a 99mTc-sulfur colloid liver/spleen image is the lateral view. This view allows for a clear assessment of the spleen's three-dimensional shape and its borders, which is essential for an accurate size determination. The anatomy of the spleen, including its location attached to the lateral border of the stomach via the gastrosplenic ligament and its extensive vascularization, supports the notion that the lateral view would provide the most information for size evaluation.