Thanks so much, Daza! I'm glad you like them. I agree, and I've found the smaller models more fun to work with. It makes things interesting because you have to be creative due to the limited number of voxels available to work with.
For the most part, I am just creating all of the models manually in the Voxel Editor by examining the 2D sprites since I don't think there'd really be any easy way to automate it - plus it's an enjoyable process getting to 3D-ify them. :) The characters/monsters have a front and side view sprite, so that helps visualize what the voxel version would look like. For some of the simpler sprites, like the flat floor tiles, I'll just import them using the "Import from PNG" feature. That is useful in general if you want to import from 2D pixel sprites since it at least gives you something to start with that you could build off and give depth to.