The WXGA+ Wallpaper size (1440×900px)
1440x900 pixels is a 16:10 resolution known as WXGA+, famous as the native panel size of older MacBooks, MacBook Airs, and various 19-inch 16:10 monitors. The 16:10 shape is slightly taller than the more common 16:9, giving a bit more vertical room, which is exactly why a wallpaper made for 16:9 shows bars or gets cropped on these screens.
Building your wallpaper at the true 1440x900 avoids that mismatch entirely. A 16:9 image forced onto a 16:10 panel either leaves letterbox bands or is zoomed until the sides are cut, so resizing to the native 16:10 ratio is the only way to fill the screen edge to edge with your whole composition intact and no distortion.
Because WXGA+ is a modest resolution, an oversized source offers no benefit; the display caps at 900 vertical lines. Resizing to this exact size gives the panel precisely what it renders and keeps everything crisp. Keep the focal point centered and away from the dock and menu bar that frame these older Mac and 16:10 displays. The entire resize happens in your browser, so nothing is uploaded, the tool is free, and your original quality is retained while the file is locked to the taller 16:10 shape these screens need. Confirm your panel's resolution before exporting, since specs differ.