Footings for gstden retaining wall

[Blockplane]

Could l have a bit of advice please. I want to build a low retaining wall in the garden, 2 blocks high, to replace the current arrangement of scaffold planks which are breathing thair last. Soil is fairly clay and well compacted. What dimensions for the footings? Many thanks in anticipation.

 

[Blockplane]

Garden.

 

[Bm101]

No expert but I believe footings are generally considered one third of wall height...
Not sure I'm on par with measuring by blocks.