Final answer:
Accumulation in the winter has the greatest influence on ice sheet size.
Step-by-step explanation:
The greatest influence on ice sheet size is accumulation in the winter. Ice sheets are formed when snow accumulates over time and compresses into ice. In the winter, snowfall adds to the ice sheet, increasing its size. Ablation in the summer, which is the melting of ice and snow, and calving at the ice edge, which is the breaking off of icebergs, can cause the ice sheet to shrink, but their impact is generally smaller compared to accumulation in the winter.