Each file must not be larger than 6mb. Most documents scanned at 150dpi are under that limit. Passports may well be more but you can using software to compress the file to under 6mb. You can either merge documents that are in the same category or upload separately.
You should scan all pages and both sides of bank statements when using them to meet the financial requirement. Some people only scan the front page when using for correspondence but we scanned all pages.
Scan every page of applicant's current passport and any other passport used during the past five years. We also scanned every page of sponsor"s passport.
Nothing is sent by post any more.
Yes, it is a good idea to start now, scan all documents to your desktop ready to upload when the time comes.