IrfanView shows only preview, not DNG image.
No.
This statement can't be true, since I provided only a 1600 x 1200 jpeg (2 Mp, along with the raw), dng4ps2 did something to create a file which showed up in IrfanView as a 3 Mp image.
...
Even if I take the raw with a 4000x3000 jpeg, IrfanView shows a 3 Mp image.
Irfanview shows DNG's well.
You can set up the viewer's options to show RAW/DNG files as
full size or
half size (default setting) image, or as
thumbnail.
CHDK RAW / DNG images has
always the same dimensions (e.g. 4000x3000), in fact
the full sensor dimension - the JPEG resolution settings doesn's influence this.
XNView also works with DNG images, there's also an option to show RAWs in full/half/thumbnail size.