As an important note to Brendon's answer, the target limitations listed in the linked resource are specific for conversions from US versions of QuickBooks Desktop to US versions of QBO. When converting a Canada or UK Desktop files to Canada, UK, or South Africa versions of QBO, the target limits are as follows:
Canada: 350,000
UK/South Africa: 500,000
Unfortunately our Canada and UK versions of QuickBooks Desktop do not offer the condense functionality so if your file is over the target limit you can complete a lists and balances only conversion to QBO which is detailed in this resource: Move your lists to QuickBooks Online
