what length do you find best for knitting socks on two circs?
24s. I think 16s are too short to keep the needle tips out of the way. If you gpo bigger than 24 than you’ll have cords tangling. IMHO.
I always use two 16" circs for sock knitting – feels just right to me 
If you’re new to the process, some people recommend using one 16" and one 24", so you don’t get the needles confused/grab the wrong needle tip when you switch sides.
Oooo, I didn’t think about that Julie. I just colored the tips on one set with a sharpie!