Apparently, two different people decided to study which countries were the most rectangular and which were the most circular. (One person per shape, not both working together in the two rankings.)
As far as I understood, a country is as rectangular as the proportion of its area that overlaps with a rectangle with the same area as the country. The most rectangular country is Egypt, with a score of 0.955. The Vatican City comes in in second place, with a score of 0.948. Funnily enough, the Vatican City is also the fourth roundest country, with a score of 0.908. The roundest country is Sierra Leone, with a score of 0.934.
Portugal is the 55th most rectangular country, with a score of 0.878. The least rectangular country is the archipaelago of the Maldives, with a score of 0.018. Maldives's score is easy to understand: there are too many islands and they're all relatively small – as opposed to Portugal, for example, that does have some islands but they're much smaller when compared to the mainland.