35 × 49 millimetres (413 × 579 @300dpi in pixels where digital submission applies). Head height should be ≈ 2/3 of height of the frame.
What background does the China 二寸照 (visa/HR forms) photo need?
White or red (marriage). Keep the background evenly lit — shadows are a top rejection cause. 小二寸 (33×48) is the passport variant — check which one the form asks for.