I was trying out the game viewer embed and it works great for posting to my blog except I don't see the checkboxes the instructions show for choosing a "small" embed option, which is what I need! How can I achieve this?

First, make sure you are using the "Chess Game Viewer" and not the "PGN Editor".
 
If you are in fact using the Chess Game Viewer, then keep in mind that the "Small" Embed option only appears after you have completed the Import of a PGN file. Let me know if this helps.