Huma41: Meshon's advice is good. Determine your use case first, then your desired size, then set the resolution accordingly. Then rough out the map such that the composition is approximately what you want and measure a feature whose size you know. That will help you determine your scale.

jugabyte: Are you in 32-bit or 64-bit Photoshop? The 32-bit version has more memory limitations. Also, you may simply be running out of RAM on your computer. You might want to version up and then flatten some of your layers in order to streamline the file.