When an encrypted item (be it one or many) is larger than 2MB, the user will receive a message stating that the item is too large to be decrypted and as a result will not be uploaded to SharePoint. The 2MB size is the default size (specified in bytes) and is configurable.
This key can only be applied if the Decryption of Encrypted Emails feature has been enabled.
Encrypted Item Size Limit
7.1 Update 4+ Email Manager for Outlook
|MSI Property Name||DECRYPTION_FEATURE_SIZE_LIMIT|
|Registry Root Key Priority||HKEY_CURRENT_USER|
|Registry Key Location||...\Contributor\EmailManager\|
|String Value Name||DecryptionFeatureSizeLimit|
|Possible Value Data||0 - *|
For details on applying a configuration setting, please refer to the article How to apply a Configuration Setting.