I tried to double the size of all fonts of all TextSymbolizers of the standard OSM mapnik style. In most cases this works fine. But in some cases the text gets clipped as show here: http://www.mytracks4mac.info/tmp/newyork.png
I do not use a python script but Tirex. But you gave the correct hint with the “buffer size”. It is also possible to change the buffer size in the map tag of the XML file. I set that to 768 and now it seems to work fine.