Answered by Mawlana Zameelur Rahman, slightly modified by the MuslimAnswers.net Team
Question: Many people who come from a Christian setting, or are otherwise new to Islamic theology ask: “Why are specification and composition impossible to attribute to Allah?”
There are a number of long replies given to this by the Mutakallimeen (scholars of Islamic didactic theology), but to keep the matter simple, we will mention the following points:
1. Something that is composite is in need of its “parts”, and God by definition is without need.
2. It is rationally possible to imagine something composite and specified (that is, which has a specific size and measurements of depth, height and length) to be “bigger” or “smaller” than those specifications. That would make God Mumkin (possible in existence) and not Wajib (necessary in existence), and this is impossible.
3. Moreover, something of this nature is in need of something external to it to set those specifications, as it is absurd that a being from the start of its existence can determine its own specifications. This would imply God is dependent and created, which is impossible.
4. Textually, God is “One” (Ahad), and something composite is by definition made of two or more parts so it cannot be one.
5. Specification and composition are qualities of creation and God is completely dissimilar to creation.
There are other answers that can be presented as well, but the heart of the matter revolves around these and similar points, and more elaboration can be presented as the situation requires.
And may the blessings and peace of Allah be upon Prophet Muhammad ﷺ, His Family and Companions. Ameen.
 absurd here means ‘logically absurd’, or ‘logically untenable’.
 as mentioned many times in the Qur’an, such as in Surah al-Ikhlaas, Verse 112:1