If you stand 3ft from a flat mirror, your image will be 3ft on the other side of the mirror, thus 6ft away. Your image is exactly the same size as you, but it appears smaller since it's 6ft away. If your twin brother stood 6ft away, he'd look the same size as that mirror image.
And, as you know, you can change how big the image appears by standing closer or further from the mirror
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