Forget Flat Earth Theory, now we have Flat Universe Theory!

[Anomalocaris]

If confuses me how anything that is infinite has a shape. So I'm just going to ignore this and go play in the mud.

The coloqual meaning of the words shape and flatness is misleading. When people say the universe has a certain shape, what they are trying to say is the fundamental characteristics of space is such that if you ignore local variations caused by disturbances like stars and galaxies, then a hypothetical flat polygon made from straight lines billions of light years in our universe would behaves as if it was etched on a surface of a certain shape. When they say the universe is flat, what they meant is the characteristics of space itself is such that if you draw a enormous triangle billions of light years on each side, with 3 laser beams for example, this triangle's interior angles would sum up to exactly 180 degrees, just like if you drew a triangle on a flat sheet of paper. If the universe is closed, then the the interior angles of this triangle would sum up to more than 180 degrees, just like you would find if you drew a triangle on the surface of a globe. If the universe is Open, then the interior angle would sum up to less than 180 degrees, exactly like you would find if you drew a triangle on a horse's saddle.

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What is the reason for doubting the infinity of the size of the universe?

I personally doubt that the universe is infinate in size because nothing in nature, to my knowledge, is mathmatically infinate and I don't see a reason as to why the universe is to be an exception.

I also view the fact that the universe is expanding to be evidence that the universe is not infinate in scope, because logically something infinate in size has nothing to expand into.
Finally, I don't see much of a reason why the universe can't simply be enormous.

The surface of a sphere is infinite. You can keep going on it and never reach an end.

It is possible to have different degrees of infinity. Imagine two infinities. One is an ordered collection of all real numbers (..... -1.000000, -0.999999999, ....etc) from negative infinity to positive infinity, the other an ordered collection of all integers (... -1, 0, 1.....) from negative infinity to positive infinity. The infinite number of all real numbers is definitely larger than the infinite number of all integers. You can prove this to yourself by seeing that you can map every one of the infinite number of integers onto a unique real number, ( 1 to 1.00000, 2 to 2.00000, etc) and you still have an infinite number of unmapped real numbers left (0.9999999, 1.0000001, 1.0000002, etc)

But you can make the smaller infinity that is the count of integers larger by also including a count of all numbers divisible by 0.5, So something infinite can still expand (......-1, -0.5, 0, 0.5, 1......) This is what is happening when infinite universe expands.

The universe can be infinite and still expand because just like expansion of the infinite count of integers, you can take each discreet subpart of the infinite universe, say a 1 light year length analogous to say part of count of integers between 2 and 3, and expand it to 2 light years, analogous to inserting 2.5 between 2 and 3.

So the universe can be infinite, and still expand. It doesn't expand into anything, each part of it just expands.