Select a root server at the bottom. Some, but not all, have a "statistics" link. Seems to be stated in qps and message size distribution, but you should be able to derive traffic volume from that.
Assuming the mean is close to the median, they are reporting ~10B requests daily with a median response size of around 1KB. 10TB daily is a little under 1Gpbs. Traffic is spiky, but this isn't particularly complex once you consider they have multiple data centers/servers. Of course, I may have misread something as daily that was hourly or something like that...
So it looks like the root namespace providers output a totally reasonable amount of traffic. Divided between the hundreds of points of presence globally, this is tens of megabits per physical host.
The servers themselves are ordinary 1 RU physical rack mount servers with 1 Gbps or 10 Gbps Ethernet. Nothing special. I'm guessing that most of the load isn't from the root, e.g.: "j.root-servers.net", but from hosting the authoritative DNS servers for .com and .net (b.gtld-servers.net) on the same box. That would surely have more traffic and much more data.