900 x 1800 is pretty large.
You can check the screen size using
get(0,'ScreenSize')
If your image fits on the screen you have to keep in mind that you have to maximise that figure window to full size, and that then you have to subtract some pixels for the decoration of the window and the amount of gray background between the image and the border of the window.
If you want that every image pixel should be represented by a monitor pixel, you have to use
iptsetpref('ImshowInitialMagnification', 100)
If then after imshow your image does not fit on the screen, you get a
Warning: Image is too big to fit on screen; displaying at 67%
> In images.internal.initSize (line 71)
In imshow (line 309)
