Opening with architraves
Architraves are the trim that goes around door frames (and windows). You'll want the door to be big enough to cover these when the door is closed. For the width
measure the whole distance from one edge of the architrave to the other + add a little more so it fully covers when closed.
For the height we can measure from the finished floor to the top of the architrave.
When the door is fitted it will be slightly lifted up from the floor, so it lifts above the architrave.
There is no rule as to how much wider and higher the door needs to be, it's really a preference or what you think looks good, other factors may come into play like distance from walls or ceilings.
Basically, if the door covers the opening + architraves you should be ok.
* 100mm is just a guide, it can be less, or more.
** You can add more height to the door, just bear in mind the distance from the ceiling to the top of the door - if there is limited space there needs to be room for the track and rollers too.
If your finished flooring is not on yet, take this into account also.