KPlayer has a unique feature that lets you choose many different options on per-file basis. These options are called File Properties and are stored along with the URL of each individual file or network location.
There are two ways to set a file property. The easiest one is by holding down the Shift key when changing a setting like video size or aspect, sound volume, video contrast or brightness, etc. KPlayer will apply the change to the currently loaded file and remember it in the corresponding file property. When it loads another file, it will revert the setting back to where it was before the change (unless the other file also has the same property set). When you load and play that first file again, KPlayer will again apply the change you made to the setting and keep it until you load another file or URL.
The other way to set a file property is through the File Properties dialog. The properties in the dialog are arranged according to their function. General properties like Name, Length and others are in the General section. Properties that control the video size are in the Size section. Other sections contain properties that affect KPlayer interaction with MPlayer and control multimedia playback, including video, audio, subtitles and advanced properties.
Select -> to open the properties of the current file or URL. You can also click an item in the multimedia library and select the command to open the properties dialog for that item.
The File properties dialog chapter gives complete details on each section and all the available properties. Also the File properties micro-HOWTO describes this feature in more detail and gives some examples.