If I remembered correctly, there are three sizes of the tags. Small, medium, and large. Please correct me if I'm wrong on this.
And there are two colours of the tags... vachetta and black.
You are only allowed to get 3 letters stamped (not including dots if you want them between letters) on a tag - though I heard some people managed to get 4 letters.
The letters also come in 3 sizes I think.
In other countries, you can only use A-Z and 0-9 but I heard in China you can have your Chinese family name stamped on tags. However, if you stamped your tag with Chinese family name you are only allowed to get one character stamped on the tag.
As for colours, there are plenty to choose.
It's probably better to go to the store and ask the SA, they will also be able to help you to choose the best combination for your tag.
And as someone has mentioned earlier, this heat-stamping service is free.