So far as I know, there isn't a maximum size. However, there are (if the following is up to date) file size limits for some price ranges, from
Enter or Change Royalty Option and List Price:
Quote:
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In addition, to be eligible for the 35 percent Royalty option, books with a file size greater than 3 megabytes up to 10 megabytes must also have a list price of at least $1.99 (1.25 GBP), and digital books with a file size of 10 megabytes or greater must have a list price of at least $2.99 (1.49 GBP).
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There are also charges to the publisher ($0.15 or £0.10 per MB) under the standard 70% option. So publishers, and Amazon, generally like smaller ebooks over larger ones.
EDIT: if you are using kindlegen see
Kindlestrip Python script and AppleScript wrapper.