1/2 drive 18"
3/8 is too small, you can always adapt down. 18" is just right, 24" can be to long sometimes. You can always put a cheater bar on an 18" to make it longer, but you wouldnt want to cut the knurled handle off a 24" to make it shorter.
Also, I've recently picked up a 18" 3/4 drive...