This is a cool idea for a puzzle, but there are a couple small things that could really improve this. For one, you can load the "size" attribute to 1, which is boring, but is technically the best way to win. I also wish that you could rotate pieces. It also took me a long time to figure out that you could place different colors on top of eachother, so that should probably be indicated somewhere. I think this could be really fun with just a bit of polish.