It would generally be longer in a large Ph D thesis.
Typically you would have a paragraph or two for each chapter or major subsection. It’s important to keep in mind that some universities put very stringent length restriction on theses Abstracts, which makes them even harder to write.
I’ve been doing the other way round – introducing first.
Hi there – generally it is easier to write the introduction last as the content in the body of the thesis sometimes changes as you get into it, making the introduction (if written first) a bit outdated.
chapters of a thesis first, then they dive into the main body text afterwards.
This means that you have to be particularly careful in wording these sections, since there is some content overlap.
You should also ask your supervisor for examples of good ones. The introduction should always follow the same basic principles – so the info here should still apply.
If your school requires a deviation from the standard conventions, then your best bet is to ask your supervisor for the specific content requirements of the introduction chapter. I tried matching your instructions with a couple of introductions to some great thesis works and noticed they were all in line with your instructions.
However, as long as you go back and revise the introduction if/as needed, after writing the rest of the thesis, it shouldn’t matter what order you do it in.
One advantage of writing (at least a draft of) the introduction first, is that you can set out your objectives clearly and keep the rest of the writing more focused. 🙂 My dissertation is regarding a spherical coordinate approach for digitally representing objects and I am writing a rough draft of the abstract, intro, synopsis, summary, conclusions, etc.