Hi John. See p. 57 in the rules - the Commanding General's distance is the same as the Brigade Command distance, so 4cm in 6mm.
You're right, the improved distances for Dashing Commanding Generals haven't been specified. Just match them with similar distances on p.57. 20cm becomes 6cm, 80cm would normally become 20cm but I have kept the distances for command figures a little generous. Normal command movement is 20cm so make it 25cm for Dashing General movement.
The rules are very good for adapting to the smaller scale figures. For me it's dining room table war gaming but I can easily play some scenarios on the kitchen table. For example, having 10mm armies (but using 15mm measurements), I was able to fight the Spittelwitz scenario on a 3 foot by 3 foot playing area - half the size of the dining room table (without adding the extra leaf) and leaving me with loads of room to stack Rules, rulers, markers, QRS's, tea cups, etc. - all very civilised!