You may want to reverse the -dset and -master datasets if you want a finer drawing. You can resample that back down to the lower resolution with 3dresample or 3dfractionize later. Alternatively, you can also open a second AFNI controller and show the ROI dataset you are drawing on as the overlay. That's initially named "COPY_...". Updates in the ROI as you draw are shown in the second controller. Still, the more important question is your goal for doing this.