[CirJohn]

While the block is active, it will spawn mobs within an 8x3x8 block area (8x8 wide and 3 high), centered on the corner of the spawner block with the lower x and z coordinates.

If, at the time of spawning, 6 or more monsters of the block's type are present within a 17x9x17 area centered on the spawner block, the spawner "poofs" without creating a monster and then waits for the next cycle.

A monster spawner is only active when a player is within 16 blocks of it.

Based on this info, you need to construct a spawning room 10-12 above your desired kill-area. Once this is complete, just take a few minutes in the kill area to figure out the furthest spot you can stand from the spawner.

Now that you've got me curious, I do intend to either do a decompile (I hate their obfuscation) or find a spawn and figure out the distance algorithm.

Edit, I just took 5 minutes to test this in creative, the player-to-spawner distance calculation is pythagorean (spherical).