when u open a photo in PS, there is no default document size,
but there are default resolutions depending on wat the image is
for.
for prints, the default is 300dpi, and for web it's 72dpi, unless
you go and change it under the options.
so if you want to print, and your image is 1800x1200, you should
see a size as 6" x 4" if your resolution is set at 300dpi, for
example. you can do this, as parka mentioned, in Image > Image
Size.
