Here are some details I would be using to build a retaining wall with galvanized steel posts up to 800mm high. This is not a guarantee that these details will be suitable for your application.
I would want to be building it in reasonable ground such as Hard Clay, Compacted Coarse Sand, or Gravel.
It is possible to build walls in poorer ground such as fine sand, soft clay and more loamy soil but specialist advice will be required.
A minimum diameter hole of 300mm dia holes at 1250 spacing (so sleepers cut in half will fit) between posts. Depth to be equivalent to wall height plus 100mm for a gravel layer at the bottom of the hole.
The minimum post lengths to be twice as long as the final height of the wall.
Aim to slope the post back at a gradient of 10-20mm for each sleeper height. Sometimes when a wall is loaded it does move slightly which compacts the ground. If you make the wall vertical and it then tilts a little after backfilling it really looks bad. When the wall is built with a back slope it just finishes up a bit more vertical.
Set the posts a minimum of 5mm below the planned finished height. It’s safer with the hard steel edges below the softer timber.
Spend some time making sure the tops of the posts are level as any mistakes really show up.
If I was using timber, rather than galvanised steel posts, I would have them on the front face of the wall so the whalings would press against the post.
Use an Aggi Pipe with a 100mm gravel surround, maximum stone size of 10mm. Continue the gravel up to just below the surface at least 100mm thick behind the wall. This is to relieve water pressure on the face of the wall. (not to drain surface run off in a storm)
Make sure the ground slopes away from the base of the wall as it helps to keep the soil around the post holes from becoming waterlogged and softening.