I didn't even realize this until recently, but US needle sizes aren't proportional at all. For instance, look at this website and you'll know what I mean.
Anywho, half of a size 9 (5.5mm) would be close to a size 5 (3.75mm). You might think at first "but half of 5.5mm is 2.75mm which would be a US 2 needle" but that will only give you a bigger headache because size 2 is so little compared to a 9. If you took the area of a cross-section of a size US 9 needle and divided that in half, you end up with a diameter of 3.88mm which is closest to (though slightly larger than) a size US 5 needle.