The Retina 15 Wallpaper size (2880×1800px)
2880x1800 pixels is the native panel resolution of the classic 15-inch Retina MacBook Pro, a 16:10 display that packs a very high pixel density. macOS scales the interface for readability, but for a truly crisp wallpaper you want to supply the full 2880x1800, so every pixel of the Retina panel is fed real detail rather than an upscaled approximation.
The 16:10 ratio is important here. A 16:9 wallpaper made for 1920x1080 or 2560x1440 is proportionally too short and wide for this Mac, so it leaves bands or gets cropped. Resizing to a native 2880x1800 keeps your composition complete and uses the full height of the taller 16:10 Retina screen the way it was designed.
On a display this dense, image quality is very visible, so a high-resolution source at or above 2880x1800 keeps textures, gradients, and fine detail razor-sharp; anything smaller will look soft against the Retina panel. Remember macOS places the menu bar across the top and the dock along the bottom, so keep the focal point centered with breathing room in those zones. The resize happens entirely in your browser, so your image is never uploaded, the tool is free, and your original quality is preserved while the file is locked to the 16:10 Retina shape. Model specs differ, so verify your MacBook's resolution.