Stupid Question: What size paper for US applicants concerning the Statement of Purpose?

I can't seem to find anything on my local consulate (Seattle) website nor on the official JET USA website about the proper paper size.

I've lived in the Philippines and Japan before so I am used to both A4 and letter size. I assume A4 but not sure. I know it's a digital document anyway but I feel like something this small shouldn't be overlooked.