A chilling image, but can it be right that the column would be 269 miles long?
The dead are marching four abreast so 1,104,890 divided by 4 gives 276,222 rows. There are 5,280 feet in a mile and 1,420,320 feet in 269 miles. If we divide 1,420,320 by the 275,222 rows it gives 5.142 feet for each row. That sounds about right. If marching men are too close together they would be constantly stumbling into each other.
So, the marching war dead of the British Empire [Britain, Canada, Australia, New Zealand, India and other smaller countries] would produce a column 269 miles long.
In the top right hand corner of the image there is a small yellow box. This gives equivalent distances for other Empire countries. For example, the column would stretch from Quebec to Ottawa. It also gives the time needed for the column to march past the Cenotaph war memorial in London. I cannot read the number but it is easy to calculate.
British Army Field Service Regulations gives a infantry marching speed of 294 feet per minute [3.3 miles per hour]. At that speed it would take 18 minutes for one mile of the column to pass the Cenotaph and 18 x 269 = 4,842 minutes [3 days, 8.6 hours] for all the column to pass.
Similar columns for the total military dead [casualty figures from Wikipedia] for other countries.
France 340 miles [127% of the BE figure]
Germany 486 miles [181% of the BE figure]
USA 28 miles [10.4% of the BE figure]
On the 1st July 1916, the first day of the Battle of the Somme, the British Army suffered 57,470 casualties, including 19,240 dead. The dead from that day would form a column 4.68 miles [7.53 kilometres] long.