Just to add to the noise, did you consider linear growth?

Unlike exponential growth, it approximates diminishing returns (i.e. tech advances become slower with time). And unlike single step, it will give people time to adapt to new realities.

E.g. 2 MB in 2016, 3 MB in 2017 and so on. 

So in 20 years we'll get to 20 MB which "ought to be enough for anybody".

But if miners will find 20 MB blocks too overwhelming, they can limit it through soft work, based on actual data.