I have read that there is no limit on file size, just on number of targets.
You can only import QuickBooks Desktop data within the first 60 days of your QuickBooks Online company start date. Your QuickBooks Desktop file will also replace your entire QuickBooks Online file and overwrite any existing information.
You must be an Administrator user on your Quickbooks Online account.
Your QuickBooks file must be under 350,000 targets (file size does not matter). To trim it down, see Trimming your QuickBooks Windows file for import. If you are over this limit, you will only be given the option to import your lists and/or balances.