The RW block and slab size is pretty much a personal preference, just like with other hydroponic mediums.

If you plan on growing small plants you could get away with 3-4" RW cubes or just one slab.

If you plan on growing bigger shrubs then the 4" RW cube might not be enough and you will need to either stack the cube onto another cube or onto a RW slab. You could also use the 6" BAB12/Extreme Big Boys RW cubes. This will work good for bigger plants but might hold too much water for small plants like herbs.

Watering rockwool properly is very important as it holds quite a bit of water and the water wicks up the cube. Soaking the RW for too long/too high will eventually cause the plants to drown as the water will wick up the cube/slab all the way up. Unless this is a very vigorous growing plant you will end up with over watering problems.

Another important thing you must do is to soak/flush the rockwool in ph set water. You need to get manufacturing impurities out of the medium as they will cause havoc on your plants. Usually water/soak the RW with php 5.5 water or use Super Natural RW soak. The RW soak also contains some food so you wont have to worry abut feeding the young plants till the first actual flood.

Arek