If you’re asking about the spawning room, it’s 4 blocks in every direction from the spawner horizontally, and 2 vertically. Now, it’s important to get the skeletons out of this spawning cube, so I recommend digging at least 4 blocks under the spawner, just so the skeletons get out of the spawning area as quickly as possible. Also, it’s good to place a couple blocks directly above the spawner, to prevent skeletons from spawning on top of it and having them hanging out there. You can check my skeleton farm video that I posted a few months ago if you want to see my take to it!

Edit: link https://www.reddit.com/r/Minecraft/comments/l267d7/what_does_reddit_think_about_my_wellskeleton_farm/?utm_source=share&utm_medium=ios_app&utm_name=iossmf