> Do you suggest I alter the VBM input so that it aligns with a 2x2x2 voxel size to match my fmri outputs?
Use 3dresample to change the covariate files to the same resolution as the FMRI data. If your original FMRI data resolution is something like 3.75 x 3.75 x 4, you may consider changing all the files (including both FMRI and covariates) to a resolution of, for example, 3 x 3 x 3. There is nothing you'd gain with a finer voxel size of 2 x 2 x 2.
Gang