Configurable scale factors (close #121)

Also changes default maximum from `4x` to `3x`

docs/config.rst

@@ -25,6 +25,7 @@ Example::
         "pngQuantization": false,
         "png": 90
       },
+      "maxScaleFactor": 3,
       "maxSize": 2048,
       "pbfAlias": "pbf",
       "serveAllFonts": false,
@@ -76,10 +77,19 @@ Quality of the compression of individual image formats. [0-100]
 
 The value for ``png`` is only used when ``pngQuantization`` is ``true``.
 
+``maxScaleFactor``
+-----------
+
+Maximum scale factor to allow in raster tile and static maps requests (e.g. ``@3x`` suffix).
+Also see ``maxSize`` below.
+Default value is ``3``, maximum ``9``.
+
 ``maxSize``
 -----------
 
-Maximum image side length to be allowed to be rendered (including scale factor). Default is ``2048``.
+Maximum image side length to be allowed to be rendered (including scale factor).
+Be careful when changing this value since there are hardware limits that need to be considered.
+Default is ``2048``.
 
 ``styles``
 ==========