Size Illusions are images where most people can't compare size of 2 objects correctly because of perspective which might be sometimes deceiving.

A ruler might come handy to check how your eyes can deceive you. Measure it yourself.

 

Shepard Tables

Tabletops on the following visual illusion are identical in shape and size! I haven't believed that either, until I measured it myself.
Shepard Tables visual illusion
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Sander Illusion

The diagonal line bisecting the larger, left-hand parallelogram appears to be considerably longer than the diagonal line bisecting the smaller, right-hand parallelogram, however, both diagonals are actually the same length. (by Friedrich Sander)
Sander Illusion
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Helicopters

Which line (red or blue) is longer? Of course, none - they are equal.
Helicopters
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Jastrow Illusion

The lower figure appears larger, however, both figures are identical.
Jastrow Illusion
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Ebbinghaus Illusion

Also called Titchener circles depicts relative size perception. Which central circle is bigger? The left one or the right one? Are you sure?
Ebbinghaus Illusion
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Ponzo Illusion

One of the best visual illusions is definitely this one. Both green lines have the same length. This Ponzo illusion uses the fact that human brain interpretes the image with perspective, however, it's just a simple 2D image.
Ponzo Illusion
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Firemen

Which soldier is taller? This is a variation of Ponzo illusion which suggests that the human mind judges an object's size based on its background.
Firemen
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