[OSM-newbies] Exporting scaled images

[Firmin,Mark [Ontario]]

First time caller, long time listener.

I've been using OSM for about three month now and am amazed at it's
potential.  I have been exporting street maps for printing, and am
drooling for more.  After reading a series of threads, I am sure that
what I am about to ask has been accomplished, but I can't find any
documentation to support it.

Here goes:  I am interested in making larger maps than OSM.org allows
and have successfully setup Mapnik to create these images, but cannot
get the scale right.  If I export using the OSM export tag, it's
perfect, but if I create a bounding box in Mapnik and export and set the
zoom level and image size, my image isn't scaled correctly.  Does anyone
know how the export algorithm in OSM works, and what steps I may be
missing?

Thanks,
Mark Firmin
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