Added option to see command block selection area with /svis cb while looking at command block, which will show cuboid or sphere depending from your command settup. In example @a[x=1,y=116,z=-40,dx=10,dy=9,dz=3] will result in cuboid and @a[x=1,y=116,z=-40,r=5,rm=10] will result in 2 spheres in appropriate sizes.