I think the most important article to estimate Uber's market size is this article, where their CEO mentions that the ground transportation market in San Francisco is around $22B and this is only for 800,000 people (http://qz.com/218717/what-people-who-think-uber-is-worth-17-...). Now multiply this by 10,000 so that you are at 8B people and you have a global ground market of 220 trillion dollars. Now that number is probably too high, as the gross global product is $100T, but the transportation market can be well in the range of $10T.

So once self-driving cars are up and running, nobody will drive their cars by themselves anymore, because self-driving cars are 5 times safer than humans driving them and for that reason, insurances will charge 5 times more if you want to drive your car yourself. Where do you order get these self-driving Google cars? You order them through Uber and then you have Uber attacking the $10T ground transportation market.(http://techcrunch.com/2013/08/25/uberauto/)