If it’s large enough to impose disadvantage on a Dex save it’s large enough to cast a noticable enough shadow to grant advantage, therefore a flat roll.
The dex save has nothing to do with size (beyond needing to be Small or larger to deal any damage at all). I assume the Disadvantage was because they didn’t see you.
Somehow I doubt it was established that the fight was at noon on the equator until that moment.
If it’s large enough to impose disadvantage on a Dex save it’s large enough to cast a noticable enough shadow to grant advantage, therefore a flat roll.
The dex save has nothing to do with size (beyond needing to be Small or larger to deal any damage at all). I assume the Disadvantage was because they didn’t see you.