I would stay quite small as crown moulding brings attention to a ceiling and can make it feel even lower and in an open concept can be an unnessessary addition. depending on style.The moulding added to this space draws you eye up and this was a 9.5' ceiling with fairly small moulding.

I would use a 5-1/2" crown..not smaller. I do not think it will make the room look smaller. I, however, always paint the ceiling a color. If the walls are a light to mid tone color, I paint the ceiling the same..this enlarges the room immensely. If you paint your walls a darker color, I then paint the ceiling either a lighter shade or a different lighter shade completely.