Two piece hijabs also known as the Al-Amirah style aren’t always a perfect fit.
If you find that the under-cap is too tight on your head, try to stretch it a bit by pulling at it or try to loosen up some thread where it comes together.
If it’s too loose the solution is pretty easy. Put the under-cap on and tighten it from behind under your hair. Place a safety pin to hold it together.
The top part of the Al-Amirah hijab can also be loosened by removing some thread where it comes together. If you want to tighten it, pull it under your chin and fasten with a safety pin. To hid the safety pin roll it into the fabric and push it behind.