Do I need steel to build a large wall as a rear boundary to my garden?

I have never known to build in a steel before but I do know you have to put in expansion joints in the wall how many all depends in the length of the wall.the best blocks would be 12x12 or 12x18 then once out of the ground you could turn it into 9 inch wall digger man would be the best at calculating that depends how deep you need to go

Depending on how big the wall is and long and wat the the wall is actually retaining, I would go for a 12/18 inch wall but wen out the ground I would do a 9inch block on flat with expansion joints every 10 block long

Hard to answer really without knowing more about the job, how big is a large wall?? How long? How high? How many skins? What's on the other side?

It depends on how much the wall is retaining. If using steel, it must be embedded in the foundation. Width and depth of foundation depend on height and thickness of wall. Allow 150mm scarcement either side of wall. Blocks should be dense concrete 7newton. Expansion joints not more than 6m apart