I think neither Endian nor iptables support that much amount of IP ranges. In fact on older versions if you add a lot of rules on Endian the IPtables crashed, and you got unexpected behaviour
And it will be slow. The best option for large IP ranges are ipsets.
http://www.dghost.com/techno/internet/banning-an-entire-country-with-iptablesipsetIPsets are faster than simple iptables rules for that amount of IPs.
Endian do have support for ipsets, but unfortunately it isn't on the GUI, or easily usable.
I've used IPsets succesfully to block whole countries (several of them) on webservers.