Can You Dress To Look Slim?

You can undoubtedly dress to look slimmmer, but if you are twenty stone, however you dress you will never look stick thin!
Dark colours do, however, help minimise your body and black is a particularly flattering colour.  White and yellow can make you look larger, so avoid unless you  are really slim.
Try to dress appropriately as well.  If you have chunky knees for example, then you will be better wearing a skirt below the knee, or else attention will be drawn to your knees, thereby making you look bigger.
Similarly if you have very flabby arms, wearing sleevelss tops will make you seem larger.
To assess your 'problem areas' (and everyone has them) you need to look at yourself in a full length mirror and assess which parts of you should be covered and which are good features which you can display and then dress accordingly, using black, dark colours or vertical stripes.  Avoid horizontal stripes at all costs !