I'm running it on a N900. I saved a picture shot by mbarcode, and the Pictures application in Maemo claims its size is 400x240. These are also the values of N900_WIDTH and N900_HEIGHT in mbarcode-qt/common.h.

The picture changed since I posted the link. Try http://http://www.qrcomic.com/39 and let me know if it works (zbar is able to decode it, this I tried already).