Gwyn's answer is correct, and I can't find any figure more precise for the exact size of Toril. All sources merely point to it being approximately Earth-sized.
The 3e Forgotten Realms Campaign Setting, p.231, contains a rare map of all Toril. The map projection being used is not stated, and the map's accuracy is uncertain, since it's specifically called "A Scholar's View of Abeir-Toril", several of the continents of which are simply called "Unknown Lands", and it is noted that "even the scholars of Candlekeep know only a little of what lies beyond the realms of Faerûn".
However, the map suggests that the continent of Faerûn is approximately one-fifth the circumference of the planet at its latitude. Given that we know the approximate width of Faerûn to be 3,500 miles (FRCS p.5), the planet of Toril is very approximately 17,500 miles in circumference at its latitude.
According to this website, the latitude of our Earth at 40 degrees North latitude is 30,600 km, or 19,014 miles. This is very close to our estimate and within a reasonable margin of error. The city of Waterdeep is at 45 degrees North latitude (FRCS p.79), so this latitude is approximately correct for Faerûn.
Unfortunately, the figures and methods used here are too imprecise to give an exact measurement. However, these estimates are consistent with the AD&D 2e Forgotten Realms Campaign Setting: A Grand Tour of the Realms p.4, which describes Abeir-Toril as "an Earth-sized planet", and the Spelljammer product Realmspace, which describes Toril as a Class E, meaning from 4,000 to 10,000 miles across, a category which includes our Earth at 7,917.5 miles diameter.