There are generally three sizes for Door Inserts, which go in the center of your door.
They are:
• 22x64 (inches)
• 22x48 (inches)
• 22x36 (inches).
Then there are sidelites, thinner glasses width-wise, that go on the sides of your door.
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They commonly come in the following sizes:
• 7x64 (inches)
• 8x48 (inches)
• 8x36 (inches)
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Images for reference:

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There are also uncommon Door Insert sizes which are:

• 20x64 (inches)

• 20x80 (inches)

• 22x80 (inches)

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It is best to measure your door beforehand to get a sense of what size would suit your door best.