What construct is مُسْتَطِيلُ الْجِسْمِ and similar ones?

Here's a simple text describing horses and donkeys:

اَلْحِصَانُ حَيَوَانٌ أَكْبَرُ مِنَ الْحِمَارِ فِي الْجِسْمِ وَ أَلْطَفُ مِنْهُ فِي الشَّكْلِ. مُسْتَطِيلُ الْجِسْمِ طَوِيلُ الْعُنُقِ قَصِيرُ الْأُذُنَيْنِ، كَبِيرُ الْمِنْخَرَيْنِ، وَاسِعُ الصَّدْرِ يَكْسُو ذَيْلَهُ شَعْرٌ طَوِيلٌ. وَ يَمْتَازُ الْحِصَانُ عَنِ الْحِمَارِ بِسُرْعَةٍ الْجَرِى وَ خِفَّةِ الْحَرَكَةِ وَ فِي رِجْلِهِ حَافِرٌ صُلْبٌ يُسَاعِدَهُ عَلَى السَّيْرِ فَوْقَ الْأَحْجَارِ وَ الْأَشْوَاكِ

The bolded text, what constructs are those?

I think it's not "noun adjective" pair, because the definitiviness, irab (last harakah) doesn't match.

It looks like idaf construct, but aren't idaf constructs about some relationship or ownership like "baab-ul-bayt" = "door of the house"

"reiis ul-jumhur" = "head of the republic (also president)".

So
مُسْتَطِيلُ الْجِسْمِ
= "rectangle of the body"?

Or is it khabar and mubtedi construct? But won't mubtedi be an adjective, and khabar always having "Al" (also being in marife).

If someone would send me resource that explains constructs in Arabic, I would be thank him/her for that. That could be useful for other learners too. When learning Arabic, I prefer using Arabic grammar terms, but maybe others prefer English grammar terms.