A spleen usually needs to enlarge about 2 to 3 times its normal size before it becomes reliably palpable below the left costal margin on examination.

However, palpability alone does not establish splenomegaly because a normal spleen can occasionally be felt, especially in thin people. Ultrasound is used to confirm size when needed. StatPearls review notes that palpation more than 2 cm below the costal margin is abnormal.