I'm afraid you got caught up in some confusing conjecture. From the inside of Sigil, the city has a measurable and traversible size; approximately 20 miles in circumference and 5 miles in diameter. Residents in the city can walk wherever they want within the city without being teleported around or anything like that. You can see the edge of the city and walk to it just as you could in the real world; it's never more than a few miles away. When you arrive at the edge you'd end up facing a solid wall of buildings without windows facing the outer edge (it makes people uncomfortable to have a view of nothing outside). You could climb up to the roof of any of those buildings and you'd be confronted with that same disturbing view; nothing. No clouds, no sky, just a featureless void. If you stepped out into that void you'd disappear and likely never be heard from again. People have been known to step off into the void, but there are no credible accounts of anyone ever making it back.