Sizes are all variant on brand and make. Each item includes the sizing in the product's description.
If an item has a true to size description, it means that if you are a true medium normally, then you would want to order that item in a medium.
When an item is described as fitting smaller, then it will recommend you to size up from your true size. Same with a description of fitting larger, then the recommendation would be to size down.
The best way to know the sizing is in the description of each item. Please read it carefully before ordering and don't hesitate to reach out to be sure.