What's better: Large or small alternator?
 

A quick theoretical question for the experts.

If one must keep an alternator, what is better--A larger one that charges on/off or a small one that is almost always charging the battery?

My car will have an almost constant electrical load, as low beams are required over here.

Smaller. Alternators are more efficient at higher loads.

You may want to look into alternator modifications if you have one outside the car that you're going to put in though, replacing the diodes with active rectifiers should give a very big efficiency boost.

It should be legal now in Poland to drive with only front facing DRLs.

LED-versions can draw as little as 8w.

You could put a switch so the larger alternator is charging only during idle and engine braking.