I am not sure with what part of the statement you have a problem.
The Delphic Expanse is brick-like in nature, a cuboid. So, if we take in rounding the actual distance across may be 1,000-1,500 ly.
For comparison, the diameter of our galaxy is 100,000-120,000 ly.
You should remember that the Xindi don't have to be all around the Expanse, just on the relative border.