There is quite a bit of information and various installs on the DRO Pros website:
http://www.dropros.com/Digital_Readout_DRO_Installation.htm
My calculations based on the travel measurements using glass scales would be:
X scale overall length of 600 or 650mm
Y scale overall length of 450 or 500mm
Z scale overall length of 550mm
You need to take a look at the mounting surfaces to make sure nothing is obstructed, I would get the slightly longer scales on the X and Y if you have the space. I would order the scales as they indicate which is overall length based on their calculations. I find that if you ask for anything else, you might not get what you want, especially when your are ordering from Asian sellers.
The X scale is mounted to the table which is the moving surface. the reader is mounted to the saddle.
The Y scale is mounted to the knee and the reader is mounted to the saddle.
The Z scale is mounted to the column and the reader is mounted to the knee.
Since the surfaces may not be even (with the exception of the X axis on the table), you need the ability to adjust both the reader and the scales so that everything is run parallel and true. This is usually done with a test or dial indicator. I mount my Y and Z scales on aluminum rectangular bar stock usually with 4 jacking screws. Scales also come with jacking screws on either end and the mounting holes are usually slotted to allow some adjustment.