Hi, Ana-

3dROIMaker has the following options, to take a subset of the top N voxels (unconnected or connected) in each ROI, if you have a "refset" of ROI locations:
-only_some_top N :after '-volthr' but before any ref-matching or
inflating, one can restrict each found region
to keep only N voxels with the highest inset values.
(If an ROI has <N voxels, then all would be kept.)
This option can result in unconnected pieces.
-only_conn_top N :similar-ish to preceding option, but instead of just
selecting only N max voxels, do the following
algorithm: start the ROI with the peak voxel; search
the ROI's neighbors for the highest value; add that
voxel to the ROI; continue until either the ROI has
reached N voxels or whole region has been added.
The returned ROI is contiguous and 'locally' maximal
but not necessarily globally so within the original
volume.

You could do some looping to subdivide things, but this isn't really based on level; otherwise, as Gang noted, you could do some "banded" ROI definition. But it would be good to have the problem specified a little more clearly.

--pt