I am new to OSM and would like to use the export function in order to use OSM (mapnik) data for a particular project.
I am only interested in data covering London boroughs at this stage.
My question is how can I tell what the maximum sized bounding-box that is allowed for exporting mapnik data at 1:50000 scale?
I don’t know the limit but the London boroughs will be some of the most densely mapped areas in all of OSM, so I would recommend you use the Planet file and extract the part you want from there or you’ll almost certainly run into the bulk downloading limits.
It’s a shame there isn’t yet a service offering OSM files for individual cities.