I have cut 3 IBC tanks in half to make six gravel beds and would like to plumb them into my existing system. I will be using an indexing valve. I would appreciate any recommendations for pipe sizing for in and out flow through the IBCs
That will depend on the pump you will be using to drive the indexing valve and what size indexing valve you get. For the lower pressure/flow the 1 1/4" valve seems to be the best choice but you still need enough flow/pressure to operate the valve.
So for the flow into those beds, you will either use 1" pipe with the 1" valve or 1 1/4" pipe with the 1 1/4" valve or 1 1/2" pipe with the 1 1/2" valve.
For the drains, well individual stand pipe drains should probably be 1 1/4" or 1 1/2" and if you combine them into a single drain line, go for at least 3" on the drain line or maybe bigger if all six beds will share a drain line or if it is going to be a long distance.