Glad you like it, most cred should go to Ivan Miranda for the core design, I just executed, updated some parts and wrote a guide around it :)

No, it's not parametric in that way. But it is possible to extend the "mill-bed" by:

* X-axis: Increase length of frame and bridge profiles. Buy longer MGN12H rails used for the X-axis.

* Y-axis: Increase length of frame profiles. Buy longer MGN12H rails used for the X-axis.

* Z-axis: Increase the vertical beams between lower and upper frame. Buying longer MGN12H rails used for the Z-axis and a longer acme rod.