Major Henry R. Rathbone, the closest man to John Wilkes Booth when the actor shot the president,  estimated the distances inside the presidential box in his affidavit given April 15, 1865.

Rathbone said the box was 10-to-12 feet long  and about nine feet deep from front to back.

The president took a seat about four or five feet from the door. Rathbone took a seat on the opposite side of the box – about seven or eight feet from the president and also seven or eight feet from the door.

Source: Henry R. Rathbone affidavit, National Archives