Lowena. I'm a whole 4 sizes bigger than you and I wear lots of patterned long tops. I think it's about the proportions of the top, I think a fitted bust then loose below is the key, it shows the (for me) slimmest bit ie bust and shoulders, then hides the rest. Also for me,making sure it's longer than than it is wide is a key point.
I have small and large patterned fabric, I think it depends on the colour of the background to a large extent, I do love large pattern on a dark/black background best.
Don't we all have odd ideas of ourselves, you're not big - I'd love to be as slim as a size 16!