EA: require size instead of __len__
Discussed [here](https://github.com/jbrockmendel/pandas/pull/3)

Change the EA interface to require `size` instead of `__len__`.

Doing this separately from #27142 makes it much simpler to explain to downstream authors what is changing for them.  After this, the rest of the changes necessary for (limited) 2D support should not affect downstream authors.