By default, the number of changes an Outlook client downloads from a SharePoint server in a single web service request or ...

By default, the number of changes an Outlook client downloads from a SharePoint server in a single web service request or "page" is 250 changed items. If SharePoint servers have reduced capacity or are overwhelmed by the size of requests coming from Outlook clients, you can change this setting to specify a different number of items to download for a SharePoint page. 

You should test changes in this setting to determine the impact in your specific environment. A page size below 15 or above 1000 is not recommended.