If a body started on one side of Sigil and walked at a normal pace in a straight line (assuming he could ignore obstacles like buildings), how long would it take him to get back to where he started? (I'm thinking lengthwise travel here, not towards the edge of the Torus where he'd fall off!) How many miles would he have traveled?
O.k., now let's suppose he went the OTHER way, starting at one edge of the Torus (say, right at the end of Suicide Alley), and travelled to the other edge (stopping right before he would fall off). NOW how many miles has he gone?
The Cage is a ring Berk, there's no end
But going round the ring, it'd have to be a pretty tough blood to make it without distractions, a trip like that would take one through the hive and the lower ward, so its not only a matter of distance.
Better ask a Guvner and maybe a Dabus, but good luck making any sense of either answer.
From a GM point of view, I'd say that Sigil is as big as you need and (barring interruptions) the round-trip takes as long as necessary for your plot
In numbers, I'd say at least 2-3 hours the long way round (again, barring interruptions), and half that from torus edge to torus edge, but you'd have to find a building and climb to the roof, to really reach the torus edge, adding more time, since the residents might object.
So, 2-3 hours, lets say 2,5 for precision, times 6 (the average walking speed of a real world human) makes for 15 kilometres around.
That means that Sigil is 15x7,5 = 112,5km.
Now, this is probably not the right way to calculate the inside of a torus, but I'm already stretching my mathematical abilities.
The above is just an estimate, I have no population numbers, no measurements, no mapscale to back it up, but if I was pressed at the table, thats the numbers I'd serve up.
Although I'd prefer to dodge the question and make the player asking roleplay his way round Sigil with a measuring tape... now there's a plothook