I want to export/save a QGIS composition of raster/vector layers to a GEOTIFF or ECW raster with high resolution (20 000 * 20 000 Pixel).
Save a complete QGIS project, rasters and vectors, to a high-resolution GeoTIFF? I don't think you can do this (at present). What you can do is export high-res images from the print composer and then georeference them.
See this feature request: http://hub.qgis.org/issues/5840
I resolved the problem by applying MapServer's Shp2img.exe repeaetadly.
MapServer is able to render a qGis project, if the project gets first exported by the standard MapServer plugin to a *.map file. Subsequently it is possible to render the project to a image file (png, jpg, tif). Unfortunately there is a limit of - lets say more or less - 10000*10000 pixel.
But by writting some programming code, it is possible to manipulate the content of the *.map file and apply sequently the shp2img.exe to single tiles of the qGis project. In the last steps the single tile images can be merged using gdalbuildvrt and gdal_translate. As output an ECW or a GEOTIFF of huge size can be produced. I wrote a small tool to automatize the process and I could export a qGis view to ECW rasters with 100'000 * 100'000 pixel! I guess there is no restriction with higher resoulutions.
As precondition FWTools2.4.7 must be installed. FWTools2.4.7 contans MapServer and GDAL with ECW output support.
I wonder why the rester export of qGis views is not a standard feature of qGis... Is it really only me who wants to export a qGis view to a well used raster format (like ECW or GEOTIFF)???
The qGis 1.7.3 Plugin "MapServerExport" can produce a MapServer (*.map) file, containing your current composition of vector/raster layers. There is a new tool TopoMapCreator (http://sourceforge.net/p/topomapcreator) on SourceForge, that is doing the conversion from the MapServer (*.map) file to a raster file (*.ecw). The tool is based on Shp2img.exe of the MapServer. You are free to define the target resolution. So a raster file with huge resolution can be generated.