The screen size is counted in pixels of the visitor machine desktop (one screen size by visit). This information can be used to modify your web site layout to take the user preference into account (the web site should be designed to be viewable in most user configurations).
In most case, human visitors navigate your site using maximized window size, so the viewable area should be near the screen size. However, in some cases, the visitors navigate your site using a non maximized window, so the viewable area could be smaller or bigger the screen size. Consequently, the screen size should be taken with care.
The screen size is computed for each visitor's visit, not for pages and hits, because it is a specificity of the whole visit.
Reporting the viewable area instead of the screen size could be more usefull, but it is not used because the viewable area depends on the browser and version used, on the screen size and on the window parameters: this would so produce a great number of existing viewable areas letting the report difficult to read.
The "unknown screen size" comes from visits in which the screen size hasn't been computed. Two possible reasons: