Sorry, I'm not good at explaining things.. I also forgot to say one needs to pick their gland weight first..
The diameter determines the end size of the glandes whether you go for a 2:1 or 2.5:1 aspect ratio. While any old plank width and pipe ID will get you the almond shaped cross section, for better glandes (I like 2.5:1) the ratio between the width of said plank and the space underneath it is what determines the exact shape. Clear as mud so far

The plank will need to be as long as you are making the roller, the longer you make it, the more glandes you can make at once. The width of the plank should be the same as the length of your chosen gland weight.
Say you wanted a 2.5:1 gland around 65g.
The best pipe to get would be a DN125 (if you are in europe).
Looking at the table, the length of gland for that pipe/weight would be 105.2mm. That's how wide you'd need to cut the plank in order to mark out the upper roller.
I hope that helps clarify things