I’ve seen different sized sites, ranging from 100kb - 1Mb+. It depends what your doing. Of course a graphicall intense site, will be larger - but then again the visitor would expect that.
The best way is to divide the sections into seperate swfs that are loaded when requested. The swf’s stay in the browser’s cache once loaded (unless you use script to force a fresh version to be loaded), so forward and back navigation shouldn’t be a problem.
Give each subsection it’s own preloader or use a uniform preloading animation for the sections.
I don’t use flash primarily for websites, so other’s will probably give your more advice, but I’m not sure how big is too big, because like I said, I’ve seen varying sized sites, and some can’t help being the size they are - but are well structured to load in stages.
Some even have games, like one I saw had a simple Mr. potato head head game that can be achieved with startDrag(); while the main site was loading.