Dark Room Trap
There isn’t much to say about dark room traps, I can only suggest ways on how to get your mobs from all over the room to your killing mechanism.
Your “Dark Room” should consist of two repeated parts: 1) Spawning pads, for all the monsters to spawn on. 2) Transportation canals, to get the mobs to the killing mechanism.
You can build your spawning pads whatever size you like, just don’t go too big as mobs will be less likely to fall off into the canals or two small, and it will reduce the efficiency of the grinder. I suggest around 14×14. I have always found this to be a good size for my grinders.
Make the dark room as large as you want, the larger the room the more mobs you will kill, hence the more drops you will receive. However this room cannot be too big as the spawning distance of mobs is limited to inside 128 blocks, so any bigger than that would be a waste.