Answer:
There is no limit of the program file size if you are using a 64‑bit system and the 64‑bit version of RFEM‑5 or RSTAB 8.
The "Limit" is determined by the computer configuration.
It may happen that the available RAM plus the virtual memory defined in Windows (free memory space) is insufficient. In this case, you can optimize the computer configuration (more RAM, more free hard disk capacity).