First of all I have so far not looked at the Pli-HD skin at all. The skin I use is Magic-HD. In that skin, most of the screens use the whole surface with a standard top part holding the screen name and the time and date, a bottom part with a logo and the active color key buttons and names and finally in most of the screens a mini-TV screen in the top left part of the screen. From what I have been reading about Pli-HD, this is started from the Magic-HD skin.
I think the screen you are talking about is the PluginBrowser screen. In Magic-HD this screen also takes the whole surface, with all of the standard parts I mentioned above. To limit that screen to a window in the order of half of the screen (both height and width) means dropping all those standard screen parts.
Such a smaller sized screen is still called a screen in the skin, and you will have to indicate for that screen where on the surface it must be located through the position= clause, and how large it must be by the size= clause. Also you will have to make sure that each of the components you wish to display in the screen has a relative position (within the window) and size which makes it fit inside the window. To enable you to do that, each of the components has its own position= and size= clause.
In the attached example skin_user.xml for the screen PluginBrowser I have dropped all the standard screen components, and just added the color buttons at the bottom of the window. Attached the example skin_user.xml file. To see what it looks like, just put it in the /etc/enigma2 directory, and restart enigma.