Is a 3mm diamond big enough to notice?

It reads as a small, clean point of light rather than a bold sparkle. On a thin chain it is visible up close but subtle from a distance.

What does VS clarity and G color mean here?

VS means very slightly included: flaws are minor and not visible to the eye. G color is near colorless, so the stone looks white in most lighting.

Which metal should I choose?

14k gold is the more durable everyday choice for a thin chain; 18k is softer and richer in color. Platinum is the hardest wearing and the heaviest of the options.

Can I order it with a different stone?

Yes. Capucinne says the design can be made with gemstones of your choosing if you arrange it before purchase.

How is the bracelet sized?

The listing refers to US sizing for the order. Confirm the chain length you need with the seller before buying, since a delicate chain cannot be resized easily.