Since you are hydro, the size of the pot is really irrelevant as all the roots will grow through the net pot and into the reservoir. I dont know what size pots you have now but a 5" should do just fine for that size. I can understand someone saying that a small 2" pot will restrict the larger roots but I have yet to see roots large enough in a hydro system to do that...you'd have to veg for months to achieve that IMHO.
The thing with rockwool is that you have to soak it in PH water prior to using it, as you should with coco and clay pellets and perlite.
The fan inside blowing across the plant would be a good idea, but again not necessary. There are other ways to strengthen the stalk without air flow.
More on Rockwool...
While rockwool is relatively easy to set up and use, it does require some monitoring and irrigation adjustment to make the best of its ability to hold high levels of moisture and aeration at the same time. Rockwool, whether it is slabs, small propagation blocks, or large growing cubes, needs to be prepared correctly by fully wetting the substrate before use. Some growers like to adjust the pH of their water to 5.5 before wetting up rockwool, but generally for small systems it's not necessary with good quality brands (unless you have a very `hard' water supply in which case acidification of the water before making up any nutrients would be a good idea). The rockwool should be fully saturated so that all of the material is wet and then left to drain. Some growers pour water into the rockwool slab before the drainage holes have been cut to make sure everything has had a good drenching, while others just pour water on or run the irrigation long enough for saturation to take place.
The most common problem experienced by smaller or new growers is over watering, and usually the grower is totally unaware that it is their irrigation program causing problems with plant growth. Flushing vast amounts of nutrient solution through the root zone in a substrate-based system often equates to plant murder - more is not necessarily better when it comes to nutrient application. This type of mistake is easy to make. After all, many new growers get enthused about hydroponics after seeing a well-run NFT or other solution culture system and assume that plants are more than happy to grow and thrive in a flooded root zone environment. However, solution culture and substrate systems are completely different and need to be managed in different ways for the plants to get the optimal root zone conditions they need.
Something else to consider when growing in any hydro system is ALGAE. I have time and time again seen algae buildup on rockwool, perlite, lava rock, clay pellets etc. To prevent this find something to cover the pot with that has an opening for your plant that does not restrict it. Regular weed barrier works wonders, as does a piece of Panda plastic. Either way cut a piece large enough to cover your pot with some overhang all around. then cut an x in the middle. fold these flaps back and place over your plant at an early age. Time and time again I see people have algae issues in hydro and then use peroxide...not entirely good for plants as it kills living bacteria that are beneficial to plants...and the problem could have been avoided by just covering the pot in the beginning....
