Use minimum size on click-to-select

[ianb]

Right now we look for a reasonable block-level element, but we don't have a minimum size. I think this results in some disappointing results.

I am not sure what size to treat as the minimum.

There are also some elements that are maybe "better" than others, such as role=article on Facebook (which will get you one comment). It might be worth going to parentNodes to find such an element, so long as the element itself is not too large (e.g., takes up most of the screen).

I'm going to do this before (or with) #1633, as I have to revisit the way calculations are done for that issue anyway (precalculating these elements instead of on-demand).