Office 365 has an item size restriction per item. Individual items above this size limit will not be migrated and normally lead to repeated errors in an attempted migration. Migration Manager for Email Archives can be configured to use a size-limit filter for an Office 365 migration. If this filter is applied, the Office 365 collection process will discover all items but only items smaller than this limit will be routed for export and subsequent import/migration. The Stage 1 screen will only show items with a size smaller than the configured limit. The items over the size limit will be left behind in the source archive. Those items can be the subject of a new migration, for example to PST, with a filter of item-size greater than the limit you chose.
Before changing/adding the item size limit, please contact Microsoft Support to check the current maximum allowed item size for your Office 365 tenant. More information on the Exchange Online limits can be found in Microsoft's TechNet online documentation at https://technet.microsoft.com/en-us/library/exchange-online-limits.aspx.
The Filter Condition value can be edited in the Console under Configuration | Advanced configuration | Migration Filters. Click the arrow symbol to the left of the checkbox for Office 365 (25 MiB Limit) policy and expand the policy to see the filter conditions. Click Edit and in the Edit Filter Condition dialog change the value to the value provided by Microsoft. Click save. The policy name can also be edited to reflect the new limit that will now be applied.