As standard, Umbraco sets the default datatype to most text based datatypes to Nvarchar(500).
Commonly this is used for selecting tree nodes from the database, which stores the data as a comma delimited string which roughly works out to about 100 selected nodes, providing the nodes are four digits long.
In older versions of the CMS, you could change the type that the data is stored in. However, version 7 has this feature removed.
If you find the Id of the node that you need to extend and run the following update statement on the umbraco database (be sure to back up before you do this!)
SET dbType = ‘Ntext’
WHERE nodeId = 123456
This will change the data type of the picker and will allow for text based string to be longer than 500 characters