I've put slightly longer blades on my Jazz and the Smart Roadster, without any problems.The method is to measure the existing blades then look to see how close the ends of the blades are to the edges of the windscreen, top and bottom ,and sides.Run the wipers to see what the sweep is.If you get a wiper 1" longer it gives you an extra 1/2" each end.

It annoys me when screenwash dribbles up the screen in the airflow over the car in the gap between the blades when you stop the wipers, so I try to reduce the gap between the two blades in the centre, as well as increasing the swept area at the sides.

You just need to avoid the blades clashing as they move.

I've also had the misfortune to buy cheap flat wipers from ebay & I agree a complete waste of money mainly sold by people just looking to make a fast buck. Can recommend these retrofit flat wiper blades http://www.wiperblades.co.uk/our-brand.php which I bought 4 months ago and performed really well during the very cold winter. Look the same as the ones off eBay, but use far superior materials and were only a few pounds more.