I recently bought a kossel mini assembled. Everything works well except it's printing too large in the x y dimension. My 20mm calibration cube is 20.7mm in x and y but is 10.03mm in z, so z is good enough for me.

I need to check it's not over extruding, but can anyone suggest any other obvious reason? This is my first delta I also have an i3 rework which runs pretty well.

I am using marlin firmware. My bed is level, my z clearance is 0.1mm across the bed surface with at z0. The part looks good and prints well.

I have tried altering the x and y steps per mm in marlin first going down to 78 from 80, then going down to 70. Each time the cube prints the same, no change in dimensions at all.

Any help gratefully received.

Have you tried reducing Diagonal Rod Length? Print a larger test print.

check out this delta calibration page