18" deep minimum. You can start at the 42", then 90 up to the 18" depth. The width should be wide enough that all the suction and return lines can lay horizontally side by side. If you have a spa or water features, the number of lines can add up quickly. A 36"-48" wide trench is not uncommon. A simple pool can be 12" wide.
Placing 2" of clean sand above and below the plumbing will help to protect the pipes.
Get a mini X rental and do it the easy way. For a simple pool one bucket width is plenty. I always prefer to pitch the lines towards the pool and come up right at the pad. The deeper the better. I use concrete sand it packs tight and water doesn't move it around or settle it if under the decking