How to design a snap/breakable PCB module?
Are there some design guidelines I should follow? What are the common
  rules (holes sizes and spacing) so the board can be snapped without
  too much strength but also not too easily?


Not really, breakable PCB's are more of an art (unless you have a mechanical engineer around with a structural FEM package). Usually what I do is try and get close to what I want on the prototype, and then adjust the hole size from there.

The more material you have between holes they harder it will be to break, if you have traces running across the pcb material between holes, you will also need to keep minimum clearance of the PCB manufacturer between holes/board edge and traces. 

With one row of holes, the PCB material will break and can cause an edge with fiber glass material and epoxy, which might be bad for handling. The dual row holes in the bottom picture makes the break cleaner. 

Also, some PCB manufacturers will do v-scoring, which makes for a clean break, but does not support traces running across the v-score.


Source: https://www.pcbway.com/pcb_prototype/PCB_V_Scoring.html