In general, using a different AFM will cause problems as it'll send a different signal to the ECU meaning it sees a different airflow.
There are some exceptions to this (eg, bigger AFM and bigger injectors to lift boostcut on a 7MGTE) and you may be able to find something that suits, but as far as I know it'd be trial and error. Just to find a slightly smaller AFM I doubt it'd be worth the effort.
Hen
PS - I've heard ST162 uses the same AFM. Again I highly doubt it's a smaller body, but you could check next time you're at the wreckers.