Resize a video while keeping the aspect ratio

Lanczos is the highest-quality downscaling algorithm — it produces the sharpest results when reducing resolution and is recommended for 4K to 1080p downscales. Bicubic is a good balance of quality and speed, appropriate for moderate scaling. Bilinear is the fastest but softest algorithm — suitable for small scaling factors or when speed is critical. For upscaling, bicubic and lanczos both produce reasonable results; neither reconstructs missing detail.

Enter 1920×1080 in the custom resolution fields. If the source aspect ratio is different (e.g. 2.39:1 cinematic or 4:3), enable 'maintain aspect ratio' to scale the longest dimension to 1920 and pad or crop the shorter dimension. For exact 1920×1080 output with padding, use the 'pad to exact dimensions' option which adds black bars. For a crop-to-fill approach, enable the crop-to-fit option.

Yes significantly — resolution scales file size approximately by the square of the scale factor. A 4K (3840×2160) to 1080p (1920×1080) downscale reduces the pixel count by 4×, and with appropriate re-encoding, file size typically reduces by 75–85%. If you downscale and re-encode at the same CRF, the output will be much smaller because there is less detail to encode per frame.