When using the Web client of CRM 2011, you only can select 250 record at once.
So if you want to execute a workflow on more than 250 records this is no option.
But how can I do something like this? Do I have to do it on the database?
There is one workaround, the outlook client.
When you open a view in outlook, you can pin the view.
When pinning the view it collects not only the data of the first page but for the entire view.
Now you can run a workflow on all records for your view. Simply press CTRL+ A and run workflow.
Enjoy this workaround.