It doesn't say anywhere within the text of Gulliver's Travels in Book Two, The Land of the Brobdingnags, about how big the country actually is. However, it stands to reason that the land mass of this place has to be incredible in proportion to England since Gulliver, himself a six-foot tall man, is comparable to the Lilliputians here. The Lilliputians were a mere six inches or the size of Gulliver's middle finger in comparison to his six-foot frame. In Brobdingnag, Gulliver appears to be a six-inch tall person in comparison to the men and women who live there.
One reason for the vague description regarding the exact location of the place and the size of the land mass is that Swift is satirizing the travel journals that were popular during this time period in England...he is poking fun of all the idealized places, people, animals, and foods they describe in the pages. There are elements of wonder in GT, but there are just as many elements (language acquisition, body language, etc.) that make the tale seem like it actually could have happened.