Can I use voxels of smaller size?

Voxel Play supports voxels of 1m in size for the world grid (ie. terrain generation and voxel address is based on the 1x1x1 scale). It uses many optimizations and occlusion algorithms to reduce vertex count, and this calculations assume voxels have a size of 1x1x1.

However, you can use custom voxel types of any shape for vegetation, trees, custom objects and items, etc. Anything that's thought as an individual element can be of any shape and size. Because custom voxel types use prefabs, you can use any geometry for these objects, and even you can have complex hierarchies or scripts in those prefabs. Check out the Custom Voxel Types page for more info.

Also, in Voxel Play 3 we introduced Microvoxels which allows you to define a custom mesh/shape for regular voxels. Check them in the documentation.

If you need smaller voxels in general for terrain, an alternative is to make anything else bigger! Make your character/NPCs/items use a greater scale and adjust the camera distance appropriately. The effect, like in cinema, will be that terrain voxels look smaller.