Initialize _sizeChanged to prevent async issues

When asynchronously initializing a map, the series of events can
create a scenario where _size is incorrectly initialized, and
therefore calls to getSize will not cause a new value to be generated.

This fix sets _sizeChanged to true upon map initialization, which
allows the following first call to getSize to work properly.

Closes #3031
Merge pull request #3052 from kshepard/fix-async-initialize

Initialize _sizeChanged to true to prevent async map initialization issues