I wanted to model a plane for my main menu at 1280x800px in size and 300dpx/in resolution. How do I modify the settings?
I don´t quite get your question. And what is dpx/in supposed to be? Do you mean DPI?
To get 1010 inch when you print with 300 DPI the image has to be 3000 x 3000px
If you have an image 1280x800px it will be 4,262,66 inch printed at 300 DPI.