Same as normal shadows
Shadows are created by blocking of light. If an object is bigger than the light source, the closer it'll get the more of the light source it'll block. Further away this effect becomes smaller until it nearly just creates a shadow as big as itself.
If an object is smaller than the light source, it is the reverse. The closer it gets to the light source, the smaller the shadow it casts. The further away, the closer it gets to it's own size.
But this isn't all. The shadow cast assumes a straight plane. 8f you make it crooked, you'll get a bigger shadow. You cast bigger shadows at sunrise and sundown.
For ease, lets grab the continent and assume the sun is directly 'above' it. The sun is bigger than our object, but it's very far away. So far away that the sunrays are nearly parallel. So much so we can treat them as parallel. If an object is caught in parallel light beams, it'll make a shadow exactly it's size. A floating continent will make a shadow as big as itself.
To determine the size of a shadow on a certain point of day, we need to use shadow trigonometry. It is too complex for me to write out now, but it's safe to say you can just use real mountains to get your answer. If you raise a mountain up into the air, it'll cast the same shadow. Just keep in mind that the mountain also cast a shadow under itself when it was standing on the ground, so it'll seem a bit bigger.
So because of parallel beams you get a shadow identical to the object. As the sun is moving, you can use shadow trigonometry to determine the shape. It'll look nearly identical to the shadows cast by real mountains, hills and plains.