Polygon layers by default do not stay the same size when zooming further out because they represent an area. The only aspect that you will see for smaller polygons when zoomed out is just the outline. If you still want to represent the feature when zoomed far out one option would be to add a <PointSymbolizer> within the sld that will turn on at x scale (defining MIN and MAX scales for the polygon and point symbolizers). Refer to this Q/A for how to set this up:
Use SLD to conditionally display points or polygons based on zoom level?Use SLD to conditionally display points or polygons based on zoom level?
