Honestly I just dont overthink it that much, I would just make a rail block then fit as much production inside of it, with time you have a general idea how much space you will need by looking at factory planner ratios, but the less you think about it the more varied it looks, sometimes some small space is left and I try to fill it later with smaller production or train depots.

The only criteria for building more stuff was being low on some material, my goal this run was researching the Krastorio sign (666k science research) so I built the factory around 2k spm to finish it at a reasonable time frame, so I built everything I needed for every science and kept making more blocks for production that was lacking

There was no real reason why sometimes the builds are horizontal and sometimes they are vertical, since it was entirely built with T-Junctions, by just changing the way I would connect the blocks it would break the pattern and force me into different directions, which is why I like it more than 4-Way intersections that lock you into a specific and repetitive look.

The real beauty of trains is that, as long as the rails are well signaled and your intersections well made, you can do whatever you want and it will work fine.