Web Part Property Storage Size Limits
For performance and security reasons, the amount of storage that a property can consume in the Microsoft® Windows® SharePoint™ Services database is limited. The default storage size for properties with a storage type of Shared or Personal is 1 MB. However, you can modify this limit in the web.config file using the WebPartLimits setting.
<SharePoint>
<WebPartLimits PropertySize="1048576"/>
</SharePoint>
If the property exceeds this storage limit, the property values are not saved and an exception is raised.
For more information about property storage types, see Web Part Storage.