to come back to this question of a battery being too big (capacity not physical dimensions).
I run my 13/60 with the standard dynamo and a 540A/62ah battery.
The car was taken for a run, the actual state of the battery charge wasn't controlled before hand - yes bad scientific practice... The route was the 'pretty' route to town via rural 'B' roads through 2 small villages with 50kph limits, a 2Km section with a 70kph limit to the edge of town. Filled up with petrol and went into town for a 'pause café' with a couple of mates fro an hour or so. Total distance each way 15kms. Same return route less the petrol station stop. No stop start driving to speak of.
Car stood for 1 week. Connected battery charger and virtually immediately showed full charge. Didn't do a volt test but the charger does do its job.
OK, so not really scientific but does show that a dynamo is capable, certainly when the car is driven on open roads, of keeping a large capacity battery charged.