The size of the consumer unit is measured in modules (the standard width of a circuit breaker), and calculating it too small is one of the most expensive mistakes to correct halfway through a job, because it usually means chasing out the recess again.
How much each element takes up
As a reference: the main switch (IGA) takes 2 modules, each RCD 2 or 4 modules depending on the model, and each circuit breaker (PIA) 1 module. A home with 12 circuits and 2 RCDs needs, for protections alone, around 18–20 modules.
Margin for future extensions
Usual practice is not to leave the unit at its limit: the number of modules needed is calculated and a 20–30% margin is added for future extensions (an air conditioner installed later, EV charging, a room that is renovated further down the line). That margin almost always costs less now, as an extra row in the unit, than replacing the whole unit with a bigger one in a few years.
Single-row or multi-row unit
Compact single-row units are fine for basic electrification with few circuits. Once you go past ten or so circuits, it's usual to move to double or triple-row units, which also make it much easier to keep each circuit tidy and labelled.
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