As far as I understand from the discussion on the comments, what you want to do is to generate an image of a large area, which is so big that does not fit on your screen and, hence, you have to zoom out, resulting in a loss of detail.
In order to export large areas, you may want to use QGIS' built-in Print Composer, this way you can control map's scale (which, as @Andrej stated, you can figure out which scale level has the detail you want to).
After that, you can export the image into a Raster or vector format, and most importantly, you won't be limited by your screen's size or resolution, as you can choose the image's resolution and dpi.
The summarised process would be as follows:
- Open Print composer from
Project\New Print Composer (CTRL+P) Give
your composition a name (it can be anything)
- Select a page size from the right panel (
Composition tab). That will be your canvas.
- Insert a map into the canvas with the following icon ![enter image
- Select the map and change its scale properties in the right panel (
Item properties tab)
- Click on export to image icon () or export to svg icon ().
- Select image's properties, give it a name and store it into a known