A standard China visa photo size accepted in Chinese embassies around the world, including USA, is 33x48 mm.
For example, these are the links to Chinese embassies, and each of them have a 33x48 mm photo requirement:
If you are applying directly at a Chinese embassy, you need to use a 33x48 mm photo.
All of our clients had no problems attaching a 33x48 mm photo to their visa application and successfully got their visas.
Note, you can do a correct 33x48 mm photo online at our website.
Many US agencies, including CIBT Visas, VisaCentral, Travisa, VisaHQ, and some others, ask for 2x2" photographs.
I'm not sure which photo format these agencies finally submit to Chinese authorities (they may crop or scan the photos),
but this works for clients of these agencies.
So, if you are applying for a China visa through an agency, and they ask for 2x2" photos, go ahead and do 2x2" photos.
Note, you can do a correct 2x2" photo online at our website, which is accepted by CIBT Visas,
VisaCentral, Travisa, or VisaHQ.