The scene size for Carrara 8 or 8.5 64 bits is limited by the available RAM. I have loaded 10Gb+ scenes on a 24GB PC without a problem. Carrara actually handles large scenes very well.

You can monitor Carrara's memory usage by looking at the Performance screen in the Task Manager. Look at how much memory is taken-up when the OS is freshly loaded, then start Carrara and your scene(s) and see how much memory they take.

When you save a large scene (like 10 GB+) it is better to uncheck the compress option. The uncompressed scene will take-up a lot more disk space but, oddly enough, it will save/load a lot faster. I guess it takes a lot longer for Carrara to compress/uncompress a large scene than just saving/loading the uncompressed scene

HTH.