[OSM-newbies] OSM export extract

[Bob Hawkins]

I wish to use an extract of OSM to show an area from within which local shops source their produce.  The area I wish to display is 30 miles (50 kilometres) radius.  I wish to export it to .png or .jpg.  It appears there is a limit imposed upon the size of extract that may be exported.  I have had mixed success so far and should be interested to know what restrictions are imposed.  Are they size of file, size of area, or what?  Also. I cannot understand how the section towards the bottom of the Export panel is supposed to work as in: Scale 1 :  (max 1 : 280000).  I can see that the (max 1 : 280000) changes with the size of area selected, but I cannot see what causes Scale 1 :  to change.  If I choose a larger or smaller zoom, the scale does not change.  I should appreciate any help to understand this whole process better.
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