How to get Big Data Sets?
Take a look at https://goo.gl/yCZvSb, a view of the Gapminder health data sets. Of the 519, you may find one suitable.

For purposes of your thesis, I'd encourage you to think of big data in terms of high-dimensional data with n >> the typical clinical trial. I'd be very surprised if you found health-related observational data other than vital statistics, such as number of births/deaths that involved even 10^9 scale data.