It depends on what I know they will be using it for. I made a king size for my aunt because she has been wanting one for her bed. She has a queen bed, but I prefer to have the quilt bigger so that it hangs nicely over the sides.
I am making a queen size for my cousin because she also wants it for on her bed.
Gifts I tend to make either a throw or twin size. I don't want to put all the time and effort into something large if they won't be using it for a bed. Throws are really good gifts.
I'm going to be making 2 wall-hanging sizes for teacher gifts. A nice token of appreciation but not super-expensive.