I have a bike with a 1" steer tube on a carbon fork.

I want to replace the stem but am not sure if I need to buy a size of 1" or something bigger. Im looking at an ITM 'big one" and found a good price online but dont know which size I need....Ive heard conflicting things about the way the sizes are listed sometimes...

so if the steer tube is 1", what size threadless stem do I need?

Can someone help? Thanks.