Hoping somebody can give me some real world numbers on the typical capabilities of 3T and 7T MRI machines in terms of there minimum voxel size (specifically in relation to there use in fMRI studies if that makes a difference)? The Wikipedia article on MRI gives a rather non committal range of 1mm to 5mm on a side, but I have been reading the term High-Resolution fMRI in a few places without any qualification being placed on it.
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 <= 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&as_sdt=0%2C9&q=high+resolution+fMRI&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.