You can't set the size of the preview cache, you can only (partly) control its size by the options you set in File Handling tab of the Catalog Settings dialog (e.g. setting the size of the standard size previews and choosing when to have 1:1 previews discarded).
Note that it is not mandatory for standard or 1:1 previews to be rendered for every image in the catalog, you can choose "minimal" or "embedded" on import, and Lightroom will then only render standard-size previews when you display an image in the Loupe, and 1:1 previews when you zoom in to a 1:1 view. You can at any time (with Lightroom closed) delete the previews cache (taking care not to delete the catalog!), Lightroom will create a new one when the catalog is next opened. This new cache will be tiny in comparison to a 'normal' cache, and if you don't render new previews for all images then these new previews will only be created on an 'as and when' required basis (i.e. when an image appears on screen in either the grid, filmstrip, or loupe). Seldom accessed 'archive' images could therefore remain without previews for a very long time.....
