﷽
Contrary to Sunnis- who claim Allah ﷻ is unlike creation, and thus incorporeal- the Mujasima[1] believe that God is a body.
Body: a being with dimensions (a height, width, or depth) stretched out in space.
We argue for God’s incorporeality with the following syllogism:
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- Allah did not begin to exist.
- All bodies began to exist.
- Therefore, Allah is not a body.
Premise 1: Allah did not begin to exist
The truth of this first premise is agreed upon between the Sunnis and the Mujasima.
Premise 2: All bodies began to exist
The second premise is true, because it is impossible for a beginningless body to exist. This is because a body is inseparable from either the state of motion, or the state of rest. So if a beginningless body existed, it would either have been:
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- Moving since eternity past
- Resting since eternity past
- Alternating between motion and rest since eternity past
All three of the above is impossible, and that which entails impossibility is impossible. So it is impossible for a beginningless body to exist. Proof for the impossibility of each category follows below.
Moving Since Eternity Past
This is impossible because the state of motion exists contingently for a body. Meaning: a body that is moving, accepts rest. Since the state of motion exists contingently, this means it was brought into existence by some specifier that selected motion over the possible alternative. And since the state of motion was brought into existence, it could not have been beginningless. Therefore a body moving since eternity past is impossible.
Resting Since Eternity Past
This is impossible for the same reason the first category was impossible. The state of rest exists contingently. A body that is at rest, accepts motion. Accordingly, the state of rest could not have been beginningless.
Alternating Between Motion and Rest Since Eternity Past
This is impossible because each alternation from one state to another is an event in time, and it is impossible for an infinite number of events to have been completed in the past[2].
Therefore, Allah is not a body
The two premises are true, so the conclusion necessarily follows. This means that it is impossible for Allah ﷻ, who is beginningless by agreement, to be a body.
[1] Mujasim: a person who believes that God is a body. This belief is called Tajsim.
[2] Given the impossibility of Tasasul.
The occurrence of an infinite number of past events is impossible, because the past is that sequence of events which leads up to, and then ends with the present moment. To claim that the past is infinite, is therefore tantamount to claiming that a sequence which came to an end is endless. And that is a clear contradiction. More on this here.
Keys to the Unseen 
There seem to be a number of ahadith from both the Companions and the Prophet (s) himself that, at least on the surface, seem very anthropomorphic (e.g. the report from Ibn Abbas that speaks of the kursi being the place of the two Feet, hadith from the Prophet (s) speaking of seeing his Lord as a man, among others). This paper [snip] even claims to establish that Imam Ahmed ibn Hanbal was an anthropomorphist, and that anthropomorphism was a mainstream belief of the Salaf. Can it be established that Allah is not a body by scripture alone, or does it require recourse to reason? Sorry, I know this is quite a large question, so if you could refer me to some reading material available in English, that would help. Jazak Allahu khair
I briefly address this in my commentary on the Kharida (especially starting from page 27 and onwards). You should also check Shaykh Abu Adam’s Sunni answers. The entire website is essentially dedicated for the refutation of the Mujasima.
I also think “anthropomorphism” is a bad translation for “Tajsim”. Anthropomorphism implies that the Mujasim believes that his god is shaped like a human, when this is not necessarily the case. Whether one believed his god to be an idol, or a cow, or a planet, or a star, or a giant being with an unknown shape and size, one would still be a Mujasim. The shape and size of the body one worships is irrelevant. What matters is that what is worshiped is a body.
Also the website you linked required a login to see the paper in question, so I wasn’t able to read through it.