How big is 1 carat diamond?

Diamonds are measued in carats, which is actually a term for how much a diamond weighs. Because diamonds can be cut with different proportions, such as depth and width, the weight is distributed differently per each diamond. For example, some diamonds can extremely deep, meaning that most of the weight is in the bottom of the stone. Regardless of the diamonds measurements, there is a general conversion chart that most diamonds fall under.

I'm assuming that you're referring to a round brilliant cut stone in this question, so Ill base the rest of my answer working under this assumption. In terms of the conversion chart, a 1 ct diamond is about 6.5mm wide.

I know that this information can be a bit confusing, and especially hard to understand in a short description such as this. There's a nice tutorial on diamond cut that discusses all of the information above in a detailed manner so you can better understand it. I added it to the "related links" that should show up under this posting.