Add size option to select and select-advanced field types.

Default to five items for multi-selects, and 0 for single-selects. According to https://developer.mozilla.org/en-US/docs/HTML/Element/select the default size for single-selects should be 0 and not 1.
Merge pull request #210 from alimony/master

Add size option to select and select-advanced field types.