That is correct. You can calculate this yourself (this mimicks the process of creating our searchlight):
sl_size = 3;
[x,y,z] = ndgrid(-sl_size:sl_size); % distances in a grid around a center 0 in coordinates x,y,z, we just pick the minimum size for a given radius, it could also be any larger number really, i.e. this is only for computational reasons
sl_mask = (x.^2+y.^2+z.^2) < sl_size^2; % this is squared euclidean distance, in your case SL is size 3
disp(sum(sl_mask(:))) % this will provide the number of entries where mask is positive
Not sure how. What is unclear about this statement?
For example, if voxel sizes are bigger for the z direction, then the distance function along the third dimension would be adapted.
mask = x.^2 + y.^2 + f*z.^2 < sl_size^2;