In slic3r under printer settings is where you tell it #1 the size of the print bed & #2 where the center of the print bed is. Then in the same menu under extruder 1 and extruder 2 is where you tell it the offset of each nozzle. If you make nozzle 1 = 0 then it will print centered and you make nozzle 2 offset however far apart the nozzles are. This of course can limit the size you print due to physical size of the heads. If you split the difference in the nozzle offsets you will gain a small amount of print space.
I don't know what your machine is but it's possible that 200 x 200 is all you can print on due to having a dual nozzle, if so then the "Print Center" needs to be set to whatever the real center of the bed is. I know, clear as mud... Maybe someone else will explain it better.