Rune
If you are using a project in Plato or Visual Studio then you can set the stack size in the Project Properties under Linker Options->Linker Options->Set Win32 stack size.
The default is documented as 50MB. See the FTN95 help file under Win32 platform->Using the linker->Reference->Interactive mode. As far as I am aware there is no imposed maximum value.
You will be aware that the stack is not used for all types of memory allocation. It may be that you can solve the problem by avoiding the use of large local arrays for example. Certainly it would be a good idea to local the particular arrays that are causing the problem to see if a change in the program design solves the problem.