Have you tried Homogeneous Reconstruction Only job on the classes you would like to “unbin”? This will output the reconstruction in the original pixel size using the angular assignment determined in the upstream refinement job.
You might wanna try info map e.g. info #1 in ChimeraX. This will show you the size (box size) and the voxel size (pixel size) of a map in the log window.
For example, in your case, if you have a mask you’ve made from your 3D classification job as volume #1 in chimeraX and your original volume (with the full box size) as volume #2, you can fix your mask’s box size using
volume resample #1 onGrid #2
That saves you the step of performing a reconstruction just to build a mask!