PVsyst doesn't take it on its own.
When in the "System" part, you define N PV modules of X area, the total area is N x X. It is not a mystery.
Now you have to foresee sufficient area in your 3D shadings scene for positioning all these modules. This seems rather logical.
What may be sometimes confusing, is that PVsyst also checks the coherency for each orientation: this check is done for each orientation independently.
I don't understand the quoted answer as applied to my example : I let PVsyst select the table size by telling it the numbers of modules per row and column. In the present case, 4x16 modules, each of them being 1.14m wide and 1.646m tall, i.e 1.92 m2. So the needed area for theses 64 modules is about 122m2. PVsyst tells : "Table area : 123.1m2 / Required area : 189.5m2". How could it be so ! I agree to the first figure but totally disagree with the second one.
As far as I understood, the PV tables are independant from the building shape. However, I checked that there is enough space around the table, in the table plane, between the edges of the table and the edges of the roof. By the way, this table design was defined by one well known fixing devices manufacturer (K2 SYSTEMS).
Thanks in advance for your help !