Designing stylesheets for variable resolution output
A common problem with map rendering is that rendered features like fonts or symbols are commonly either too small or too large depending on the resolution of the device you are targeting. For example, most Mapnik stylesheets are designed with normal, moderate resolution computer displays in mind. For print graphics or for more modern and high resolution displays like on many mobile devices existing stylesheet design can lead to inscrutably small fonts, symbols, and line symbolization.
Using scale_factor to scale up or down rendering sizes
Mapnik since >= 2.0.0 has supported an optional argument for rendering known as scale_factor. This is a fractional value that is used as a multiplier throughout the rendering pipeline. It defaults to 1 meaning that all features will be rendered at the exact pixel size specified in the stylesheet. But if you pass a scale_factor of 2 then symbolizer properties like line width, line dash-array separation, svg and png icon size, font size, halo size, glyph spacing, and many more will be scaled up by 2. You can also design your stylesheet with oversized defaults and scale down by passing a scale_factor of .5 or even .1. This scaling down approach can have advantages so that things like PNG/JPEG icons are sharp and correctly pixel snapped at their default size and can be scaled down without loosing quality. The opposite of course is not true - while Mapnik as of 2.2 is able to scale bitmap icons up the quality will not be as good.
Gochas
If you are using scale_factor ensure you are using the latest Mapnik version. The Mapnik 2.2.0 release, for example, implemented many subtle fixes to increase the quality of rendering when using a custom scale_factor and future releases will surely fix more issues as they become apparent.
Usage
From python you can pass a custom scale_factor as the third, optional argument to mapnik.render:
im = mapnik.Image(width,height)
scale_factor=2
mapnik.render(map,im,scale_factor)
im.save('scaled_up_2x.png')
The C++ API also supports an optional third argument for scale_factor:
double scale_factor = 1.0;
mapnik::agg_renderer<mapnik::image_32> ren(map,image,scale_factor);
ren.apply();