Can I fit larger wheels and tyres to my Mercedes-Benz B-Class?

I recently purchased a 2005 Mercedes-Benz B-Class and would like to upgrade it. Can the wheels and tyres be upsized safely?

Will fitting smaller wheels to my car improve ride quality?

I have a Mercedes-Benz ML-Class, with Airmatic suspension. The car is on 21-inch wheels at the moment. Would it make a big difference to the ride if I fitted 19-inch wheels?

Will changing the wheel size make the ride more comfortable?

I have a 2009 Ford Focus with 17-inch alloy wheels. What is the standard size and, if I changed to the standard size, would the ride be more comfortable?

MINI Hatch (2007 - 2013)

Great performance from Cooper S, much smoother and more sophisticated than first generation, capable of travelling distances in comfort.