Typical and minimum voxel sizes for various field strength MRI machines?
This is a slightly older post, but the Wikipedia article is basically correct.  It is a non-committal range of 1mm - 5mm.  Typically high resolution fMRI will probably be &lt;= 1mm on a side, but is typically going to be higher field scanners (>>= 3T) or using slightly different sequences than the standard fMRI sequence.

Best thing to do would be to do a quick survey of the literature (e.g., https://scholar.google.com/scholar?hl=en&amp;as_sdt=0%2C9&amp;q=high+resolution+fMRI&amp;btnG=) and get an intuition from there.  I think what you will find is it will depend a lot of sequence, sequence parameters and main magnet field strength.