I've done all in this series and from memory, you are yet to merge Hispaniola. A good trick to do would be to merge Hungary and Ukraine, Burundi and Rwanda. But the ultimate troll would be to do Azerbaijan and Turkey.
Italy/Vatican, Italy/San Marino, Luxembourg/France, Liechtenstein/Switzerland, France/Monaco and Andorra/Spain would all make impossible ones. But for bigger countries, you can try Norway/Russia, South Africa/Lesotho, Brunei/Malaysia, Myanmar/Bangladesh or Eritrea/Djibouti.
According to Wikipedia, yes, it would be, and it wouldn't be that close. Even if both Quebec and Texas left, the US of Canada would still be bigger than Russia. Even without Alaska, it would be bigger (though if Alaska returned to Russia, then Russia would be bigger).
Canada and the U.S. as one will have to do.