I typically take them through food size 1.75 to 2 lbs and start from 8" to 10" in spring having the ready for harvest by following November. When starting from smaller 1" fish in spring they can get to be just shy of the 8" to 10" size needed for following spring. Starting with larger and not getting as big, I think 60 to 90 days is all the time needed if growing during the summer. Should be on lower end of that range. A 4' x 4' cage will be OK I think if water quality is kept good. I have done on round and rectangular cages but not with purpose of comparison. Info should be out their in a masters thesis somewhere. Would look into to ways of getting water to move through the cage and economy of scale will hurt you on that. Fish would be increasing in weight by 4 to more than 8-fold so for every lb of gain assume about 1.25 lbs of gain. I would use pellets that are formulated for salmon to promote faster growth.

If no air double size of cage at least and keep out where water flow is not obstructed by shallows.