current best practice for layout dimensioning is ems and %, relative dimensions that auto adjust to window size, screen resolution, and user preference enabling a single site layout to work in everything from a handheld device to a widescreen monitor, and permitting disabilty friendly display. All fixed sizing is outdated
If you are asking about images and having one site with a higher image quality than the other - it wouldn't matter. No matter what setting a monitor is at, even the finest "resolution" on the screen, it will only display the image at 72dpi. Any other size will only make the file size larger, and take longer to download.
If you are wanting a solution that will allow a visitor to see a website with larger size (not dpi size - but the final image dimensions shown on their monitor), you could create 2 separate CSS styles and give them the option of viewing either site. This way you are providing both, but not forcing one on the visitor.