Is 20ft of 3/8th too long (volume-wise)? What dimensions do others use for a HERMS coil?
If you just want to maintain the temp in Mash tun, 3/8 will do just fine.
If you want to raise mash temp with it, I would say that 1/2" would be a better choice.
20 feet is long enough.
It is all about how much volume you can pump in certain amount of time.
If you want raise temp fast, you also need to have appropriate heater in your HLT.