I usually make a pizza dough for 3 pizzas, from 500g flour, but once I made a double sized portion, with doubling everything in the recipe and I was not totally happy with the result. Since then I have read that scaling should work the way I did it, with proportional amounts of everything, so that should be fine, but I was wondering: if the recipe says X minutes of kneading, then if I double the amounts should I knead for 2X minutes or the same amount? Are there any good advices for scaling?
When it comes to kneading, scaling partially depends on your physical capabilities. It's a physical activity, you may not be able to manipulate double the dough in the same way. If you have the arms of a gorilla it will probably take far less than double the time, if you are on the weaker end of the scale it may take you longer than twice the time as you might have to get it partially kneaded and then split it into halves to complete, or you simply tire and slow down.
The message here is to knead for a result, not a time. That's actually one of my primary pieces of advice when bread baking in general, and not just kneading. Knead until you get the texture you are looking for, no matter the time.
I don't think that using set amount of time for set amount if ingredients is the way to go. There are too many variables that would affect the result (type of flour, temp of water, type and amount yeast, temperature in which the dough is resting etc).
I would just give the dough extra time for it to work on itself (kneading is just speeding up the natural process) until the dough will be rested and "as you like".
IMHO the best way to do pizza dough is to leave it in warm place for 20 minutes before kneading. Then kneading until the dough is firm, giving, and smooth. Then leaving for another 10 minutes under damp towel (or plastic foil)