By "slide," do you mean an image file? If so, load the image into the Viewer window and change the duration in the upper left hand corner. Now drag the right-hand side of the image clip in the Timeline and it should drag as far as you've set the duration.
FWIW, you can set the default duration for still images in FCP's preferences. But that will only affect images imported AFTER you've made the change.
-DH