According to Amazon, you can upload manuscript files up to 650MB. However, since Amazon will charge delivery costs ($0.15/MB), if the publisher chooses the 70% royalty option, you may want to keep the file size as small as possible or break up the book into several parts.
Even though many KDP authors/publishers had Calibre generated mobi files accepted by KDP, you may want to use Kindle Previewer/Kindlegen to be on the safe side.