If the data file has more than 350,000 targets, the file will need to be SuperCondensed
first. It can then be converted to QBO.  
The SuperCondense service will shrink the size of your data file to almost 50-80% of
the original file size which boosts performance, offers stability, speed and keeps you
under the under the 14,500 name limit for Pro/Premier, helps you upgrade to new
versions of QuickBooks easier and also enables you to convert to QuickBooks Online
with ease.
Use a professional third-party conversion provider for a solution or PM me for more information.