The export to Excel/CSV button has a property Maximum number of rows
in the modeler. By default this value is set to 1000, but you can increase this if you want.
Note that increasing this value can also increase the memory consumption (of the Mendix server) when a user presses this button, so use with care.
Thanks for the quick responses.
A workaround in your case (considering it's so simple: one field) is to build a java action that simple outputs all the values to a string and then store it in a filedocument.
The excel button has a property with the maximum number of allowed rows.