I believe 10 digits is all you can use right now because of 32-bit implementation limits that have not been removed yet in AR System. Hopefully you can achieve what you want with nextid=1000000000.
Even if everything is 64bits on 7.6.04 SP2, i.e. AR server, database, plugin, etc., we still have some 32-bits limitation ?
I've tried to find information about that limitation on ARS documentation, so that I can explain that to the customer, but I've found nothing...