A quick Bash script told me that currently there are 1385 articles linked from the online TOC (
http://plato.stanford.edu/contents.html ), and that the uncompressed size of all the HTML pages is ~127 MB.
If you are going to create a single EPUB, I doubt eReaders will open it, due to its size. Tablet apps might be ok.
In case, do not put every entry in the TOC. Just put "Letter A", "Letter B", etc. linking to an XHTML page that contains links to the entries. Probably a 2 level TOC ("A" > "AB", "AC", ... | "B" > "BA", "BE", etc.) would still be manageble by most reading systems.
EDIT: from a quick test, the EPUB containing all the articles is around ~40MB. My iPad opens it, but it is very slow at opening it from the library, and to navigate the TOC. My Kobo Glo is simply not able to even open it (or, more precisely, seems still trying, after ~5 minutes)
EDIT: with a bit of labor, one can remove the junk and get a decent EPUB, still slow to load from library, but at least the navigation is not untolerably slow. See attached screenshots.