It's simply not possible when working with pixel-based data. Any resampling operation causes loss of information and thus quality - downsizing interpolates neighboring pixels and sizing up again will have no knowledge what the original, non-interpolated pixels looked like. Simple computer math getting in the way.
I figured would lose something, but was wondering what would be the least harmful. Tried keeping the pixel dimensons what they were, and changing resolution to 600 dpi which reduced physical size by 50%. Then experimented with reopening the file next to the original — hard to visually see much of a difference on screen. The image will be used as a decal/wrap for the side of a large store cooler so probably should be ok.
File size and the number of bites Photoshop has to deal with is directly related to the number of pixels and the color bit depth.
DPI is related to devices and the intended pixel density.
For example ink jet Printers can print using different size pixels up to their maximum resolution. They use their smaller high resolution ink drops to paint in your images larger pixel size. By varying the DPI setting without changing the number of pixels in your image you can print it at different sizes. All the images will have the same number of pixels. Howver they have different size pixels. For example 300DPI, 600Dpi etc Their pixel density differ.
Display run at their native resolution have a single pixel size a single DPI. All of your large print size image's pixels can not be displayed on you display are once because your display can not display that many pixels are once. You need to scroll around the image to see you images actual pixels. Zooming out lets you see you whole image by reducing the number of pixels used to display your image.
Some Tablets and Laptop these days have high resolution displays some with over a 300 DPI. Still their Screen are to small to display all the pixels you get from a good digital DSLR. My DSLR deliver 8MP and 16MP images. So actually the Old IBM 22.2" desktop 204DPI T221 Display which has been discontinued can display my 8MP cameras image for it displays 9.2 MP 3840x2400 Pixels. The new 4K TV displays will come close to that many pixels but because they will be large the image they display will have a low DPI resolution.