How to get correct scaling

I have designed a 3d model with a square thats supposed to be 1cm³ to get the scaling right. Unfortunately, i have to print at a shop and they have asked me how to get the scaling right. How can i help him get the scaling right?

I have designed a 3d model with a square thats supposed to be 1cm³ to get the scaling right. Unfortunately, i have to print at a shop and they have asked me how to get the scaling right. How can i help him get the scaling right?