What makes you think it's not 1024 x 768? That's a lot of pixels and probably won't fit on screen in Photoshop without scrolling at 100%. Check the image size...
Photoshop displays pixels without regard to resolution. 1024 x 768 @ 300 ppi looks exactly the same as it does @ 72 ppi or 1200 ppi. Physical size doesn't exist until you put the image onto paper.
In addition, if quality is important you'll do far better by exporting to PDF and opening that in Photoshop to rasterize it.