In a Mapnik XML Stylesheet the MinScaleDenominator indicates the minimum map scale at which a symbol or feature should be rendered.
One inch to the mile maps have a scale of 1:63,360, so setting MinScaleDenominator to be 63360 would mean that the symbol or feature would only appear when you render at a scale showing more detail than 1 inch to the mile.
Working out the map scale is a bit tricky. If you are rendering an area of 5,000 * 5,000 Spherical Mercator co-ordinates (ie. 5km * 5km) into a 1024 * 1024 pixel image then your map scale will be about 5m/pixel. Each pixel is assumed to be 0.28mm on your computer screen, so the scale value for your map would then be 5,000 mm / 0.28 mm which is approximately 18000.
Below is a table which shows the relation between zoomlevel and scale and which may help to see at which zoomlevel a feature is rendered or not:
Example: If you set MaxScaleDenominator inside of a style-file (eg. osm.xml) for a feature to 18.000 (without setting a MinScaleDenominator) this feature will be rendered on all tiles for zoomlevel 15 and higher.
If you want to know more about map scales, look at this link: ScaleAndPpi.