I’m not sure what it might be now, but I believe it really depends on the store you plan on selling it as there is no hard maximum used throughout the industry.
According the the Nook spec (circa 2010), it says max file size is 300kb for reflowable. They may have upped that, but that’s what their latest spec says.
I’m sure iBooks is higher than that, I think I saw someone say its something like 1-2GB, which I haven’t tested but is on the higher end of the spectrum. I wouldn’t be surprised if it was indeed that high or higher seeing how screen resolution on their products is increasing.
So if you want to sell your ebook in every store, you have to limit yourself to the store that has the smallest file size limit, or just plan on selling it in certain stores that allow for larger file sizes.
I do believe the file size limit is much larger for the fixed layout format as they usually are much heavier in images/video/audio.
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