You'll be fine using a 6-qt. mixing bowl if you're doubling a recipe!

Which pie recipe are you making? I think that it would work to double the recipe in terms of proportions, but I'd be worried you might run out of room in your mixer, as meringue grows in volume when you mix it!

Yes, I regularly double meringue recipes and they work out just fine! Just make sure the bowl you use is large enough to hold the volume once the eggs reach their fullest!