Load your video, pick a preset resolution or enter a custom width and height, and export. The resized file is ready to download.
In almost every case yes — locking it prevents stretching or squashing. Only unlock it if you deliberately want a non-proportional resize.
Yes, though upscaling cannot add real detail that was not captured, so a heavily upscaled video will look softer than genuinely high-resolution footage.
1080x1920 for vertical Reels/Shorts/Stories, 1080x1080 for a square feed post, and 1920x1080 for standard horizontal YouTube uploads are the common defaults.