The thing is size shouldn't diminish if you select and breed for it. By selecting out your biggest/meatiest stock to breed off of, your going to work towards a bigger next generation. Same thing with rate of maturity so you get good fleshing. If you have a cut off date that you want to process by say 16 weeks, then use that as when you'd evaluate your biggest/meatiest birds and then cull the rest from the breeding program. Then pair them off and collect eggs, hatch them, monitor your chicks for size and meatiness, then re-evaluate at 16 weeks, breed, and the process continues. So long as you maintain high standards you'll get this off the ground.
You shouldn't have to keep reintroducing the CX back into your lines if you keep separate lines and select for the biggest/fastest growing birds. That's not very sustainable because your still having to bring birds in, and keep raising CX to fuel your lines.
The ALBC has an outline on their site for selecting for a meat bird. It says how to gauge your stock, and what to look for. The one thing Id be most worried about is the hens ability to lay a consistent and reliable quality and quantity eggs. Id be crossing to large egg dual purpose heritage breeds to ensure you don't lose any quality as far as your eggs go, otherwise your going to have to keep a number of hens back to ensure you get enough eggs to hatch for the following season. Something like a Barred Rock, Australorp, Delaware or even an Orpington.The goal is to think longer term and the CX will give you immediate size but not something Id want to have to keep working back into my lines every other year. At that point you might as well just have the CX and a layer flock.