Your approach is wrong , unfortunately. The standard you quote is for pile design, not retaining wall design.
Most geotechnical textbooks have a section on retaining walls and the steps required to determine the length. There are likely some examples of calcs give on this forum if you search.
For embedded retaining walls you need to determine your factor of safety against over turning.
In your case, the pile diameter is only really dependent on the structural capacity of the wall. You need to check the moment and shear force applied to the wall against the structural capacity of the wall.
You might also resize the diameter to increase stiffness and minimise deflections.
Page 201-C760 gives detailed steps on how to design the wall. It does refer to Eurocodes 7 requirements (DA1C1 and DA1C2), however these are just factors that you apply actions, and parameters etc. The fundamentals of what you need to do is laid out for you in detail.