There are two types of brushes in Voxel Max: Cubic and Spherical. These brushes can be used by the various tools and modes in Voxel Max, and will help you build large objects faster than placing single voxels at a time.
Brush size is defined by its radius, or the center voxel plus the number of voxels to reach the edge of the brush. So brush size 1 places a single voxel. Brush size 2 places a block or a sphere that is 3 voxels wide (1 voxel for the middle + 1 voxel radius on all sides). Brush size 3 is 5 voxels wide, size 4 is 7 voxels wide, and so on up till brush size 32, which makes a 65 voxel wide sphere or cube.
To select a brush, tap either the sphere or box icons. To set their size, press and hold on the desired brush and a menu will appear. You can drag up or down to select the size. For easier control of the sizes, you can drag up to the center of your screen, then drag up or down to set the size further away from the edge of your device.
When using a brush, the center will be placed on the voxel you press, so tapping on the floor will result in half the brush being invisible under the ground plane. You can make the full brush visible by using Control Points or the Guide Plane.
Learn more about the features and tools in Voxel Max with this video.