Each set can actually have max 131072 elements: https://github.com/NethServer/nethserver-blacklist/blob/master/root/usr/share/nethserver-blacklist/load-ipsets#L111
For now the limit is hard-coded. Extending such limit, could eventually bring to performance loss.
Also bear in mind that many ipsets has duplicated IPs. I think that you did the right choice: do not use extremely large ipsets.