Thank you, both okh and Puybaret.
I'm currently going through my furniture library to see if deleting some items will help free up some memory in the program. Also, when I save it, I'm clicking on "Save and compress"; after doing that it seems the file is now 29.7 MB. That is smaller than 4.5 GB right? I'm not sure the which is bigger, megabytes or gigabytes.
I have replaced version 5.1 on my computer with version 5.2 so there's no chance of my using the older version for it now.