Welcome to the SeeMeCNC Community

After recalibrating my machine with 626pilots firmware...I proceeded to print my 20mm cube again.

It seems like every side is approximately .3mm too small. This was printed with ABS.

Even with ABS shrinkage, this seems like an aweful lot. Where would I begin to go about figuring out what is wrong?

I do not know what values you have in your configuration.h #define DELTA_DIAGONAL_ROD
This value controls the length of the arms and this in turn controls the size of printed objects.
I believe that if #define DELTA_DIAGONAL_ROD is made smaller, the printed part is printed larger.
The scaling within your slicer can also be adjusted.

I have the tricklaser arms, and its set to 269. I will try decreasing by .5 increments and see what happens.