How big could I grow a plant in a 13cm round pot only a few inches deep filled with clay pebbles. Could I grow a 1 foot plant in that and then transplant into a bigger pot to flower?
How big? That depends on a lot of variables. You just risk your plant becoming root bound. But when you transplant, you can cut/score some of the root mass to trigger new growth in the new pot. Here's a pic of one of the best examples of a solo cup plant I could find.
Ok don't worry about a net pot what size on average do you think you could grow to in a 4inch rock wool cube. Basically what I want to do is keep my plants vegging for as long as possible in a 4 inch cube before transferring to clay pebbles in my hydro system for flowering
As long as the plant is supported and it is provided the proper nutrients and environmental conditions, in vegetative growth it will just keep growing. You should do fine in 4 inch rockwool. I'm making the assumption that you are moving your vegged plants to flower, then taking new clones in 4 inch rockwool, and then want to move them when the flowers finish. Am I correct? How big they grow depends on strain genetics, training methods, and environment.
That's an option but I actually have a mother plant I take cuttings from for now. But ye perpetually take cuttings in 4 inch cubes and get them vegging while a batch is flowering. I haven't used 4 inch cubes before so wasn't sure what the roots in there could support and if they will transition ok to clay pebbles when I flower