Retired in August this year and now lives in other places. Recently, Gansu Provincial Social Security Bureau requested pension qualification certification. Who can tell me how big a photo I need?

You should follow the size, pixel and format required by the Social Security Bureau. Generally, there will be detailed verification notice. Please read it carefully before you do it, otherwise you can't verify it. Different regions have different requirements, including photo verification and fingerprint verification. There are also various photo verifications. Just look at the notice.