If your looking to make a block like a slab, where you can fit 2 blocks in the same block space, good luck I have no idea lol. But if you want to just make a block smaller, download blockbench, you can make loads of things on that program. Then you need to learn how to change the "bounding box" dimensions for your block. I suggest watching some YouTube videos on blockbench and bounding boxes
If your looking to make a block like a slab, where you can fit 2 blocks in the same block space, good luck I have no idea lol. But if you want to just make a block smaller, download blockbench, you can make loads of things on that program. Then you need to learn how to change the "bounding box" dimensions for your block. I suggest watching some YouTube videos on blockbench and bounding boxes