This is the size in code, data, and heap that applications require. On linux, you can check the size of "text" (code), "data" and "bss" with size a.out (replace a.out with what you call). Similar tools for Windows.
As for the needs of the heap, it is more complex, for example, loading a complete Boeing 747 model (each nut, rivet, bolt, seat and button on the “TV remote”) into Autocad takes up much more memory than a model of two bolts with corresponding nuts through two plates of some metal in the same Autocad system - the same thing as downloading the latest Ken Follet novel to a word processor is clearly different from downloading your letter to a water company to complain about a water leak on your street. But a rough estimate using a typical use case (for example, a model of something small, but still complex enough to make sense).