FastPictureViewer supports a special mode that automatically rescales images to make them fit in memory. With this feature you can easily display very large images even if your computer resources would not normally allow it.
This function was tested with images up to 466 megapixels so far (21,600 x 21,600), one such image taking 1.8GB of RAM once decompressed in 32bpp mode and causing every image editor and viewers we tested to choke badly.
This feature is configured simply by telling the program the maximum memory a single image is allowed to consume, everything else is automatic.
The program ships with this feature enabled by default and set to a maximum 112 MB of memory consumtion per image (this means that image above approximately 30 megapixels will get downscaled to fit within this memory constraint). Images below that threshold are not affected and there is no hard-coded limit on the maximum image dimensions the program can handle.
Incidentally, the default setting allows images of any size to fit within the constaints of a 128MB graphic card. The memory threshold can be configured from 2MB to 1GB per image in the program options, see Menu->Options->Resource Usage.