Epson GT-6500 EN - Manual (Page 126)

The physical distance of the offset lengths and scanning lengths
depends on the current dpi and zoom settings. For example, when the
main scan and sub scan lengths are set to 400 dots and 640 dots
respectively (at 100 dpi and 100%), the physical lengths are 4 inches
(10.16 cm) and 6.4 inches (16.26 cm) respectively.
Relation to other commands
The reading area is reset to the maximum value when the
ESC R (Set resolution) command or ESC H (Set zoom) command
settings are made. Make those settings before the scanning area is
specified.
The scanning length determines the number of pixels (dots) to be read.
The actual image data can vary from 1 bit per pixel to 8 bits per pixel
according to the data format setting made with the ESC D command.
The main scan value should always be specified as a number that is a
multiple of 8. The offset lengths can be any integer value.
3-32 Command Reference