It is recommended to choose a blanket that is between 8-12% of your bodyweight. If this overlaps between different weights then it is down to your preference.
Some people may want a very light pressure outside the 8-12% range and others a heavier pressure.
Weighted blankets are designed to be used by 1 person only. The sizes are based on your mattress size NOT your duvet so they may be a little smaller than expected. You could use a single, double or king size weighted blanket on any mattress but it may look better draped over your bed when matched with your bed size.