Michael Schaffner (aka Schnapps) posted a document elsewhere on the internet - "Statement showing the number of principal articles of clothing and equipage purchases at the depots of Philadelphia, New York, and Cincinnati since May, 1861" - found in the Official Correspondence. The copy that I printed off finally surfaced from the morass that is my desk so I can offer the following additional information on hospital flags.
The statement indicated that there were two kinds of hospital flags as well as ambulance flags. The line entries for each indicated the following:
General Hospital flags: 262 were purchased at Philadelphia; 101 at New York; and 200 at Cincinnati; for a wartime total of 562.
Post and field hospital flags: 698 were purchased at Philadelphia; 201 at New York; and none at Cincinnati; for a wartime total of 899.
A separate line entry for Ambulance flags indicated the following:
2,500 were purchased at Philadelphia; 401 at New York; and 1,750 at Cincinnati; for a wartime total of 4,651.
Now separately, I found in my copy (reprinted) of "Civil War Battle Flags of the Union Army and Order of Battle" originally published as 'Flags of the United States Army carried during the War of the Rebellion, 1861 and 1863", on page 40, the images of all three types of flag. They all feature the yellow field but otherwise have some differences in size and other design features.
The General Hospital Flag is 9 feet by about 4 and 1/2 feet (along the fly/staff edge). Centered on the yellow field is a large green H, about 2 feet high and about 18 inches at the base and the top.
The Post and Field Hospital flag is identical in appearance, but is about 6 feet 4 inches long and about 3 feet 8 inches along the staff and opposite edge. The green H is about 13 inches across and 1 foot 6 inches high.
The Ambulance flag and the guidon used to mark the way to the hospital(s) is yellow, about 30 inches long and about 15 inches along the staff - edged with a 1 inch green band on the three sides.
It is noted that these patterns date from January 1864. There is no information given regarding the patterns used before this date.
Robert A. Mosher