The HD+ Wallpaper size (1600×900px)
1600x900 pixels at 16:9, often labeled HD+, sits neatly between the 1366x768 of budget laptops and Full HD. Plenty of mid-range notebooks and some monitors use this resolution, and a wallpaper built to exactly 1600x900 fills those panels pixel-for-pixel with no upscaling and no bars along the edges.
Because HD+ is a fairly modest resolution, there is nothing to gain from a wildly oversized image; the screen tops out at 900 lines of vertical detail. Resizing to this precise size hands the display exactly what it can show, which keeps edges clean and avoids the mild softness of on-the-fly downscaling from a much larger source. It also keeps the file light and quick to set.
Composition-wise, treat it like any 16:9 desktop: keep the focal point roughly centered and clear of the corners where the taskbar and icons live, since on a laptop-sized panel those elements occupy a noticeable share of the screen. Bold, clean subjects hold up better than busy detail at this resolution. The whole resize runs in your browser, so your image is never uploaded, the tool is free, and your original quality is preserved while the file is fit to the 16:9 shape HD+ screens expect. Since panel resolutions differ, confirm yours before exporting.