What we have is documented with references to sources on the Forgotten Realms Wiki, and several people have created globes of Toril with the map of Faerun projected onto them.
Toril was comparable in size to the planet Earth, with an equatorial circumference of about 25,000 miles (40,000 kilometers). Its polar circumference was slightly smaller, at approximately 24,900 miles (40,100 kilometers)
The Forgotten Realms Campaign Setting on page 4 states:
Abeir-Toril is an Earth-sized planet dominated by a large continent in its northern hemisphere as well as a number of other large landmasses scattered about its surface. This northern continent is called Faerûn in the west, Kara-Tur in the east, and Zakhara in the south.
That means if you know the latitude of Waterdeep and Chult, you can estimate the various location's latitudes (although these maps do not state the type of projection that is used).
From what it looks like to me, Chult is somewhat north of the equator. There are several maps that show the equator south of Chult, for example here, or, as a globe projection, here.
The only canon map from a TSR/Wizards of the Coast published source that shows latitude and longitued lines is this one from 1996's Faiths and Avatars.
It is not clear from it what degree the various lines represent, but you can clearly see the equator (on the widest point of the globe section) running through a good bit south of Chult, as in those other, fan-made maps.
If you also consider that all known maps end in the north in section of ice wastes, you can conclude that that would be the region of the north pole, and grid your longitude lines accordingly, and that is what most of the fan projects appear to have done, with relatively comparable results.
