Hello Yana,
For fixel-basd analysis (FBA), it is pretty straightforward. If you check the current FBA documentation (https://mrtrix.readthedocs.io/en/latest/fixel_based_analysis/st_fibre_density_cross-section.html), you’ll see that step 7 recommends upsampling only if original images resolution is above 1.3 mm voxel size. HCP data have 1.25 mm voxel size, so no upsampling is required (notice that you also need to drop the -voxel_size 1.3 in the last command of step 10).
Other than that – notice that your device would not be able to run FBA analysis of data with 1.25 mm with an 1.25 mm template, as it doesn’t have enough memory. You’ll need 64GB+ for that (see discussion in thread Fixel Analysis Computer Question). The thread mentions, however, the possibility of downsampling the template to reduce memory requirements (if you don’t have access to a computer with a larger memory).
Good luck,
Oren