While the voxel size and registration method might affect the
blur size one chooses, nothing affects the choice more than
what one plans to do with the results.
In this case, your results are being handed to an FSL ICA
program, and it is likely that the software was designed with
some expectation about how much blur might be applied
before the software is run. It is quite possible that the
authors even expect no blurring to be applied.
Find out what the software expects, and let that influence
what you tell afni_proc.py to do.
- rick