You can use finer resolution for the segmentation than the input image. See detailed instructions here.
Do you also have a CT image corresponding to the SPECT image? If yes, then it would make sense to choose that as a master volume for the segmentation, as most likely it has sufficiently fine resolution.
I would recommend option A (crop volume), because you can crop away a large part of your volume and so you can have a high-resolution volume, without increased memory usage.
Option B. (use specify geometry button) would work, too, just would make things slower. For your data, you would need to use an oversampling factor of about 4.0, and since the physical extent of the volume will remains the same, the memory usage (and computation time for most operations) will be 64x more.
You can read a short summary of the module on the right side. In short, “Crop volume” module is for cropping volumes (3D), while “Crop volume sequence” module is for cropping sequence of volumes (4D data sets). Therefore, as I wrote in my last post, you will need to use “Crop volume” module.