Let's sort'em out.
The gap the zombies have to pass through is initially 3 blocks high.
On the upper part, i place an upside down bladetrap.
On the floor, i put halfblocks to raise the zombies up to make them pass slightly higher.
And of course, an electrical fence to increase the time they spend in the bladetraps (which also reduce how long the bladetraps will last without repairs, of course).
Result:
Taller zombies like Demolishers, Cops, Wights, snow zombies and on occasion some regular sized ones will be hit in the head by the bladetraps, while smaller ones will pass through (although taking damage from the electrical fence wire).
A very crude and rude sorting machine.
I have not managed to trigger a demolition zombie with this setup yet, although a sample size of 20 is a bit small to determine if it is perfect or not.
edit: A slight bonus with the setup in the video, keep in mind that the highest DPS from a bladetrap is under the centre of it.