Kdenlive is too large on my screen. I cannot make it smaller.
This usually happens when too many widgets are open. Each widget label takes a minimum amount of space in width. Close some (e.g. via the View button) and consider using layouts (View -> Save Layout As).
I want to trim videos without re-encoding them. How can I do this?
You cannot do this with Kdenlive. (Please try Avidemux instead.) The reason is that, for splitting, files need to be treated in a very different manner (the file itself needs to be edited, whileas Kdenlive renders frames into a new file). See also: , 
I want to apply an effect, for example a watermark, to the whole project. What is the best way to do this?
Create a new project with the same project profile and import the project on which you want to apply the effect to as a clip (Project -> Add Clip). See also: How to: Add a Watermark in kdenlive on Vimeo.
My monitor plays distorted images, or generally something it really should not.
Please check your Settings -> Configure Kdenlive -> Playback settings. Try to disable OpenGL if it is enabled, or use a different driver. Kdenlive may need to be restarted.