Probably about a tenth of the width the 82nd and 101st were scattered across Normandy!
If you have Google Earth or just Google maps.....look up Fort Bragg, North Carolina and look for the drop zones....named for famous parachute assaults.
That will give you some reference for what is available there and the lengths of the DZ's.
The C-17 is designed to drop 102 Paratroopers and 8 Equipment Bundles....so that would be 51 per door.
Drop speed is about 130 Knots I suppose.....so watching that video and counting Chutes and marking the time....should provide a way of guessing the distance for a single pass drop.
Combat drops are lower than training jumps....normal is 1200 feet AGL.
Fort Bragg Drop Zones
There are other DZ's used off of the Fort Bragg location....that SF and other Spec Ops units use for training Ops.There are a pretty good number of them, large and small, everything from Sicily (2.5 miles long and a half mile wide) to St Mere Eglise (a 6 second DZ . . . . . in a Casa)
Here's an alphabetic list:
Gela Drop Zone
Holland Drop Zone
Nijmegen Drop Zone
Normandy Drop Zone
Oran Drop Zone
Rhine Luzon Drop Zone
Saint Mere Eglise Drop Zone
Salerno Drop Zone
Sicily Drop Zone