How to achieve passing dropdown list parameter to filter SSRS report with two options

I am new to SSRS. I developed a report and added few calculated fields. It is showing all the data fine. Now I want to add filter using dropdown list parameters. The way I want to do is when user select SHORTNAME from the list, all the names from NAME column with length less than 6 are displayed and if the users select LONGNAME from the list, all the names from NAME column with length longer than 6 are shown. Please suggest how can I do this?