There are several ways of doing it. The fastest is using Common length, by dragging the end of one of the selected notes while holding CTRL.
![](https://daw.one/shared/images/Common_note_nength.gif)
Another way is using Musical funtions > Length > Set all to.
![](https://daw.one/shared/images/Musical_length_function_set_all_to.gif)
Length has arguments available, so it's possible to create a macro. For instance for setting up a shortcut to always use Set all to 1/8.