I'll have to measure it. Ironically, I've been running a Spektrum hard shell 4s 5.0a G1 dumb battery in it all along. It fits easily, I'd guess the 1/10 Nut might fit a small 6s even, say 4.0a? I'll get you some measurements later today, after work.
Funny story, about a year or so ago, HH put the Spektrum "8s Power Pack", or whatever they call it on sale for $199. S2100 dual "smart" charger w/ 2x4s 5.0a G1 "smart" batteries. I told my brother about it, but was too broke(and smart) to buy one for meself. Well he suprised me by buying 2 and gifting one of the power packs to me. Cool, awesome, thanks bro!!

Fast forward about a month or two, the S2100 gave up a channel, lights up, registers.. no function from that channel.. so, the batteries are "okay", not great, and thankfully since they're G1 and not G2, they have a standard balance connector. So, I'm able to charge them on a standard charger at least. The G2 Spektrums are what I call "dumb" since they cannot be charged on anything other than a Spektrum smart charger. That's dumb IMO.