Below I will attempt to compare the dimensione by picturing how an observer from each would see the "same" objects.

First we're starting with a view of objects in a way we are used to seing them, with length height and breadth. Represented here are some basic figures, prisms, pyramids and speres. A horizon line an grid are also left in for perspective purposes.

On page 69 in the book, the sphere attempts to describe a cube such as the one pictured at left. "Behold this multitude of of movable square cards. See, I put one on another, not as you supposed, Northward of the other, but on another. Now a second, now a third. See, I am building up a solid by a multitude of squares parallel to one another. Now the solid is complete, being as high as it is broad, and we call it a Cube."
 
 

This was of course very difficult to comprehend for the flatland inhabitants, as for in flatland things appeared much like they do in the picture at right. You may notice it is the exact same image, transfered to two dimensions. Note however that if you were indeed a flatlander, looking from the corret perpective you would notice no more than a line, provided you weren't excersising sight recognition. 

As described by the sphere, "what you style Flatland is the vast level surface of what I may call fluid"