Hello @rob10,
Currently, the import bank transaction feature can only accommodate up to 350 KB.
I suggest exporting your transactions from your bank in batch. Then, import the exported files to QuickBooks Online one at a time.
While I'm creating a report regarding this issue, I suggest sharing this to our developer by going to the Gear icon, choosing Feedback. Our developers will consider all feedback and suggestions for product update ideas.
You can always get back to us here in the Community if you have any other questions.