Next, create another, smaller sector (say, 128 units on each side), somewhere a little away from the first. Once done, right click and hold inside it, and drag it over to meet the north edge of the first - don't overlap though. If you can't do this, it's because Doom Builder's switched to Linedef mode - hit S to go back. Then, create another square sector inside the first, in the exact center, and this time just 64 on each side.