each block is roughly .5m ^3 which i think is a great size as is with everything at its current scale. it would be nice, however, if the grid was made smaller so we can make smaller blocks and add in some extra block shapes and variations so we can build some really detailed structures. So instead of making a stair block, you can make individual stone steps and make each a different pattern. Some extra block shapes would be nice. For example, i'd like to have a 1/4 cylinder shape. something like that might make a nice curb.
Regarding the slabs, the slap block is a block so cannot be freely rotated. i made a suggestion last month about rotating blocks and essentially spawning a custom grid so that you could, for example, build buildings at different angles. It would be complex to code but certainly a nice feature but there are other higher priorities in the game right now. In an older version of the game, there was a slanted block which was essentially a slab type block but at a 45 degree angle. It's not craftable anymore but I'd like to see that block type be re-introduced. i wonder why it was taken out in the first place.
Of course, you could just not go the block route and do custom building with wood beams and planks. You can make those any size you want, probably make them as small as a tiny block and those do rotate freely. I haven't messed with that system too much but essentially build a saw bench and craft wood beams and wood planks. Its harder than the block system but is very flexible. I think you're limited to the types of materials you can use however. It would be nice to get all the materials ported over to the beam/plank system so you can create, for example, long marble slabs for window sills and such. 