Feature Add: Portfolio Shortcode Size option

The Jetpack portfolio shortcode was hardcoded to use "large" for the size. I have added the "size" option for the shortcode and it works with the typical "thumbnail" "medium" "large" as expected with the wordpress documentation. The size of the thumbnail and the cropping is the typical settings for the gallery.

Please see this code for the simple fix.

https://github.com/jsidabras/jetpack/commit/68922884eb54309cac2d83a13ad6ef24e25d65cf#diff-2da4a1879ab00cd88d001db523ff3ed2

Feel free to use this if you feel this is a feature worth having.

Start with the linked commit and inspect its diff for the portfolio shortcode implementation. Compare the existing shortcode behavior with WordPress’s documented thumbnail size values, then verify that selecting each supported size produces the expected gallery thumbnail and cropping behavior.

Written by the indexing model from the issue text.