My dear aunt Catherine wouldn't allow me to use the word can't. "Can't never did anything!" she would say. My mother often repeated the same thing. I wasn't allowed to quit without at least trying.
Apparently much of DesignCAD's code still dates back to windows 3 or perhaps MS-DOS days. It has been patched so many times the patches have patches. I think this is the primary cause of problems in the video display routines. But I also suspect that somewhere in the code are variables, arrays, whatever, that still have 16 bit or 32 bit limits. It all needs to be rewritten.
With 64 bit variables there is no reason (other than processor speed) why the program won't work with file sizes in the tens or hundreds of gigabytes. But I haven't been able to get it to work with files much larger than about 750 megabytes.
Like you, I have 32 gigabytes of RAM (the highest speed RAM I could get), and only about a gigabyte ever gets used for program storage for the operating system and user programs, and a few terabytes of unused drive space, so there should never be a file size limit.
Back in the early 2000s DesignCAD was limited to file sizes of about 60 megabytes or less. I ran into this limitation with several drawings I was creating. Files were not paged to hard disk while they were being edited. Somewhere along the line file size limits were expanded with 32 bit code, and then 64 bit code. But something is preventing the program from working with gigabyte size files.
For comparison, Photoshop would work with gigabyte image files decades ago by paging them onto the hard drive. It was VERY slow, but it worked!
DesignCAD used to give a warning when it was running out of usable RAM, but it was accidental and subtle. One by one various commands would just stop working. You could select the command from menus, tool bars, hot keys, etc., but nothing would happen. At that point you had to save your drawing, shut down DesignCAD, restart it and open the drawing again. This makes me think that the program wasn't releasing resources after they were used, so the operating system ran out of RAM. Closing the program released the resources.
When I have reached the ~750 megabyte limits with DesignCAD files I have tried shutting down everything - turning off the computer and rebooting. But even this doesn't allow me to add to a ~750 megabyte file. But in the latest releases I don't see commands stopping working like they used to. You can draw new objects but they won't be in the file the next time you open it. If you paste objects into a file some parts of the pasted objects just don't appear in the file.
Note: I have been working on a CAD model of a guided missile cruiser for about 14 years. It includes almost all details of the real ship down to 3/32 inch diameter (screws, rivets, etc.).
It is currently in four files of 200-350 megabytes totaling a bit more than a gigabyte. I have succeeded in pasting two of these files together to create a >500 megabyte file. But if I try to paste a third file into the composite drawing only part of it gets added, and some parts are lost. The resulting file is 720-750 megabytes.
Just loading files this size takes many minutes, and OpenGL Gouraud shading can take up to 90 minutes (4.3 GHz 6 core i7, full speed DDR1300 RAM and fast USB3 hard drives). This is what I use to test the limits of new DesignCAD releases.https://www.okieboat.com/CAD%20model.html