Paul,
I wondered the same thing when I converted my large route I started back in the TRS2004 days to HD and the route went from 334 MB to over 2 GB!
With that in mind, an exported route will be compressed to half that size when exported to a .cdp file. I'm not sure what the exact size limit is with a .cdp file and I'm afraid we'll reach the file-size limit for that format. The issue is that we can write large .cdp files but they can't be imported again. I ran into this with TS12 and with T:ANE. T:ANE was a bit more forgiving but TS12 Content Manager crashed to the desktop in seconds of barely ingesting the file. That file was about 750 MB at the time.
Knowing the exact size limit will help, but do you want to cut your route apart to save it in order to be able to import it later? This would surely be very inconvenient and really inefficient.
I use .cdp files all the time as interim route backups and for final route backups. The interim backup is used for when I'm going to merge or do other drastic changes to an existing route and the just in case backup has come in handy more than once. Needing to restore the two parts separately as two routes and then merging them back together will definitely be more than annoying.