SUMMARY OF MY POST:
- Don't use Word for your thesis; on the Mac, it's unstable for long documents.
- LaTeX is robust, and particularly functional for scientific theses and others needing to incorporate equations, tables, and vectorized and non-vectorized graphics.
- I've not used Scrivener, but you might want to consider it if your thesis is mostly text.
- Regardless of which program you choose, universities have very strict formatting rules, so see if you can get your hands on a template made for your university in your program of choice (for mine, there was a LaTeX thesis template available, though I had to update it).
My thesis was not atypical for one in the sciences: ~200 pages (double-spaced), subdivided into chapters, sections, and subsections, with numerous equations, tables, inserted vectorized and non-vectorized graphics, and over 100 references. It also had an automatically-generated table of contents.
I initially tried writing it in Word-for-Mac, but after it got to about 20% of its final size, the file began crashing (the program would have to close the document, which meant I had to save it continuously)---when you get beyond a certain document size, the program simply becomes unstable. Plus it doesn't handle inserted graphics robustly, and its equation formatting capabilities are limited. [I suspect the Windows variant is more stable.]
I thus switched to what I should have used in the first place: LaTeX. On a Mac, the easiest way to do this is to download the entire MacTex package from the tug site (there’s a link to it on the TeXShop page); this will give you the latest TeX distribution, as well as a very nice front-end editor (TeXShop) and reference-mangement program (BibDesk) (after completing this, do “check for updates” on both of the latter two).
Note also that universities typically have very strict thesis formatting rules, and at most universities someone has probably written a TeX thesis template. You should talk to the other graduate students -- they can probably tell you where to find it, and let you know of any modifications that need to be made to it to bring it strictly up-to-date (in case it was written a while ago).
It took me a couple of weeks to get comfortable with the program (and learn its basic idiosyncrasies), and another two weeks to become fairly fluent, but after that it was off to the races.
I've not used it (it wasn't available for the Mac at the time) but, if you want a WYSIWYG TeX editor, you might want to try BaKoMa TeX.
Having said that, I understand that many novelists use Scrivener. Because of that, and because Scrivener was originally written for the Mac, it might not have Word-for-Mac's stability issues for long documents. If your thesis is mostly text-based, you might want to give that a shot. [I don't know how Scrivener would deal with graphics, equations, etc., or how easily it interfaces with reference-management systems (I understand some people use Zotero, which in turn requires either Open Office or Libre Office: https://danielvreeman.com/using-scrivener-for-writing-scientific-papers/). ] You can Google 'writing thesis in Scrivener'; I, for instance, found this: https://tacet.wordpress.com/2014/06/02/using-scrivener-to-write-a-dissertation-why-im-glad-i-did-and-what-i-would-do-differently-next-time/ It would certainly be easier to learn than LaTeX.