Albert Barnes Commentary Ezekiel 48:17

"And the city shall have suburbs: toward the north two hundred and fifty, and toward the south two hundred and fifty, and toward the east two hundred and fifty, and toward the west two hundred and fifty." — Ezekiel 48:17 (ASV)

The city being 4500 reeds square, 250 reeds are marked off north, south, east, west from the city land.