First, you should try the same size as of your normal shoes. For instance, if your normal shoe size is 9, then you should first try the roller blades of the same size.
Typically, they must fit you, however, the fit can also differ from manufacturer to manufacturer. So, you just need to select a design of your choice, and ask the storekeeper to bring different sizes that can fit you. If you wear 9 number shoes in daily routine, then you can ask him to bring 8, 9 and 10, so that you can check each of them, and choose the one which perfectly fits you.
Once you feel that the roller blades fit you, you can kick the heel on the floor. This will assure that your heel is well placed in the heel section, also known as heel pocket, of your roller blades.
If your heels are well placed in the heel pocket, then you should lace the roller blades up and stand on your feet. Now you should make sure that the heel is still in the heel pocket and your toe should hardly be touching the toe box of your roller blades.
For the final test, you will have to bend a little to bring your body in skating stance. Once you do that, your toes should pull back slightly, and your toes should not be too tight or too loose. If they are loose, then you must try the small size, and vice versa.