Quick Rockwool Question

I have only used rockwool for seedling propagation and wanted to know what is the appropriate size cube to use for growing to around 3 feet. Is this even advisable with Rockwool? How about using the slab?

Thanks

the 4" cubes are what I use about 1/3 submerged in a bed of Hydrotron and 3 ft be no problem. The 6" hugo blocks are even ,ore stable but in my setup are overkill.