See Every Tweak With Bigger Filter Gallery Previews
Once you’ve chosen a filter from the Filter Gallery (under the Filter menu), you’ll probably want to spend some time tweaking the settings. If that’s the case, you’ll also need to see a larger preview of your work so you can really see the effects of each little tweak. You can do that by clicking on the up-facing triangle button, to the left of the OK button. This hides the center column (the list of filters) and expands the Preview pane into its space, giving you the full preview experience.