Actually there is a way but you have to keep in mind that the smallest entity is a Voxel. I.e. if you "resize" the result will be looking best if you can divide the current size by 2 (if you have e.g. a width of 3 and you want to resize it, you will end up either with 1 or 2 Voxels in width, not in 1.5).
Menu: Modify > Transform > Scale
Write e.g. "0.5" in the field for the axis you want to scale down.
Another approach could be to do it outside of Cubicle. This only works if you are happy with your model already and you simply want to align the size to other elements in a game or so. You can free scale your model in Blender and other tools.