Hi - I have an eGo-C Upgrade that I bought by accident thinking it was a regular eGo. To be honest, I think the "Upgrade" idea was a gimmick they put out before the Twist. It has two voltage modes, but isn't a true variable voltage. I haven't noticed a significant difference between the two modes, and since it's not very clear where you are in the cycle (in the unregulated mode the voltage changes as the battery discharges), I just use the constant mode like a regular eGo.
I don't have a ruler within reach, but most of the vendor sites list the lengths of the various mAh
batteries.