Since this is just an emergency drain line in case of an minor overflow of the sump, I personally wouldn’t go to crazy on the size.
The diameter needed would be dependent on the flow rate you’re trying to handle, the slope and length of the line through the crawl space, whether it could achieve full siphon, etc. Too many variables.
If I were doing it, I’d probably only use a single 3/4” or 1”. It’s not intended to handle all the flow of the aquarium. If something that catastrophic happens, you’ve got bigger problems.
I’d probably incorporate a vertical standpipe at the point the water drops down into the crawl space to relieve any back pressure from air trapped in the pipe. I believe conventional plumbing in homes works that way with a vent pipe so sinks and showers don’t have to create a full siphon before draining efficiently.
Just my input...