You just keep up'ing your size of rockwool cube.
Start with a 1x1
Then transplant the 1x1 into a 3x3
then if you need to you can transplant the 3x3 into a 6x6 if you need to, You dont have to remove the plants from the cube they are in you just place that cube into the new larger cube.
And as for the pH tester do a little reading mate as you can get them anywhere.
Start reading as much as you can now because if you dont know how to use rockwool properly it can kick you in the ass!
J