"optimal" paper size suggestion ridiculously small

[hholzgra]

Allowing for larger areas to be rendered, and so lower zoom levels, leads to the minimal paper width and height almost always being below the hard 10cm limit.

This can lead to severe problems when an output format with street index is selected (Issue #63), but also does not really make that much sense in general.

So the "optimal" / "default" size suggestion probably needs to be redone ...