Alas. 0,0 means nothing now, my sky to lava tower stands abandoned. At this point bigger is better as far a s surface area goes up to 144 x 144. I use terraces and water to funnel the mobs down to a 16 x 16 drowning / cactus ( for spiders ) trap. My terraces are 8 wide with channels at least every 3 blocks that feed to the next terrace down and a 2 block drop off so the mobs can only go one way. Keep in mind, no spawn within a 24 radius of you. Eliminate all alternative spawn points in your 144 x 144 area with torches, water, or by building your trap out that far out.
Read through this thread for a great design and youtube tutorial on how to build a modular trap and grinder, it will work as big or small as you want to build