There is no maximum file size, but you need to keep an eye on the market. If your target is as many readers out there, you are almost bound to ePUB2 and need to support e-Ink readers. That means low performance hardware with limited hardware. So, not many bells and whistles.
If your target is a full blown ePUB3 with audio/video for a tablet, the restriction would be the free space on the tablet. Not that I think your audience would be happy if you take a lot of space though...
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